Contagion of Interests ~ A Case of Trilobite Arches

Interests can be insidiously contagious.  Though you may contract only a mild version of the enthusiasm that grips the person who initially exposed you, the damage has been done.  This posting is a cautionary tale.

Recently, my sister and her husband returned from a vacation in Venice with tales of architectural marvels, including trilobite arches.  They were certain this term would excite my paleontological persona, prompting an immediate mental connection with trilobites, those extinct arthropods of the class Trilobita.  With references on these arches from one of their guides to the city, Venice from the Ground Up (2008) written by James H.S. McGregor, chair of the Department of Comparative Literature at the University of Georgia, they launched me on several days of research, the drafting of this posting, and descent into . . . .

A bit of context may be in order.  Though these folks are not paleontology fanatics, some time ago I infected them with just enough of my passion for the science that they now go out of their way for something fossil related.  A case in point – during an Adirondacks sojourn, they stopped by Leeds, New York, so they could walk the town’s bridge, constructed of Becraft Limestone, a wonderfully fossiliferous Devonian stone replete with such treasures as crinoid stems, gastropods, and trilobites.

To the arches.  McGregor uses the term trilobite arch in describing aspects of the Basilica di San Marco (St. Mark’s Basilica) and the Ca’ d’Oro (House of Gold) on the Canal Grande, with particular attention to the latter.  He writes that the Ca’ d’Oro features trilobite arches in the loggias on the western façade’s first and second floors.  (The numbering of the floors is tricky, a ground floor sits at the canal’s edge.  A loggia is a covered gallery, open on one or more sides.)

To establish what I’m focused on here, below is my rendition of a trilobite arch drawn from those on the second floor of the western façade.  (I drafted it using Inkscape, an open-source vector graphic editor.)

Among the defining elements of this Gothic arch is not the outer edge (the extrado), which in this case is simply pointed, but rather the inner one (the intrado) which is divided into three spaces by two distinct protrusions on either side.

The full western façade of the Ca’ d’Oro is pictured below.  (The image, taken by Didier Descouens, is reproduced under Creative Commons Attribution-Share Alike 3.0 Unported License, and appears at:  http://en.wikipedia.org/wiki/File:Ca%27_d%27Oro_facciata.jpg .)

McGregor describes this façade as follows:

Springing from two pilasters and supported on five columns, a complex tracery of stone distinguishes the first story.  The lowest area of the tracery is a series of trilobite arches, with frames that are steep and doubly curved and inner circumferences lightly marked with the outline of three partly overlapping circles.  In the open areas between each of these adjacent arches, the stone tracery outlines four-lobed openings.  The tracery comes to a point above each of these openings to form small trilobite spaces. . . .

The arcaded opening on the floor above is a more compact and delicate version of the one below.  Five shorter columns and two pilasters support six trilobite arches that terminate in a much simpler and more distinctly geometrical tracery above.  (p. 119-120, emphasis added)

I love that the “small trilobite spaces” he describes on the first floor (and, for that matter, those on the second floor) are upside down.

In fact, this kind of arch (right side up) appears widely in Venice.  For instance, Basilica di San Marco is replete with them.  The picture below of the southern doorway at San Marco shows a wonderful set of these arches framing the windows above the doors.  A separate photo focuses on the arches.

With these images in mind, I asserted that this term –  trilobite arch – applied to the delicately flowing structures on the Ca’ d’Oro façade made perfect paleontological sense.  Whoever named this arch, I believed, could only have been doing so with some knowledge of fossil trilobites.

I didn’t mean the connection was as simple as there are three divisions in each – the arch has three lobed spaces and the arthropod trilobite had three body lobes.  Rather, it was in the arrangement of those three lobes that the name made sense and here a bit of specific paleontological knowledge came into play.

Most people upon first encountering a fossil trilobite conclude that it is so-named because it has three stacked body parts dividing the animal from the head down to the tail – into cephalon, thorax, and pygidium.  The photo below of an Elrathia kingii (1 inch long) has been annotated to show those three divisions.

But that would be incorrect.  Further, that arrangement wouldn’t match the trilobite arch.  In point of fact, the names Trilobita and trilobite as applied to this extinct animal derive from the three lobes into which the body is divided longitudinally with these structures running the length of the body, two pleural lobes straddling a central axial lobe.  (Among other publications that discuss this source of the names is Trilobites by Riccardo Levi-Setti, 1993, p. 8.)

The longitudinal aspect of the lobes signaled to me a clear bond between trilobite arch and the trilobite arthropods.  With an open space typically extending down from the trilobite arch, one is presented with a structure whose name architecturally and paleontologically makes sense.

Only the connection is an illusion, it’s purely serendipity and the product of my paleontology delirium.  In my enthusiasm, I’d seen a connection between Venetian architecture and these fossil animals which is purely a coincidental, misguided product of a consuming interest.

Perhaps McGregor simply needed a different adjective with which to describe these three-lobed arches; he may well pronounce the word trilobite with an accent on the second syllable (tri-LO-bite).  Yes, the term trilobite arch is used elsewhere by others but very seldom.  A Google search came up mostly empty.  I realized that I’d seen intent where there wasn’t any, a point driven home when I found that, as far as I can tell, architect and renowned historian of Venetian architecture Richard J Goy in his book on the construction of the house (House of Gold:  Building a Palace in Medieval Venice (1992)) never once uses the term trilobite arch.

I’ve concluded that this kind of arch is perhaps most often labeled a trefoil arch.  (I should admit that Goy in the passage on page 145 of his book providing a succinct description of the western façade only uses the term trefoil arch a single time.)  From Oxford Art Online (available by subscription) comes this definition of a trefoil arch:

A triple arch composed of three sections of a circle, arranged scallop-fashion, the central being the highest.  It may be pointed or round.

In his American Architecture:  An Illustrated Encyclopedia (2002), the late Cyril M. Harris defines a trefoil arch as follows:

A pointed arch whose inner surface is struck from three centers; the shape of the arch is determined by the position of the centers and radii of curvature; has a projecting cusp on each side.  (p. 338)

(Harris was professor of architecture and professor of electrical engineering at Columbia University.)

Use of the term trefoil arch appears to have deep roots.  Among the older material I turned up is The Principles of Gothic Ecclesiastical Architecture:  With an Explanation of Technical Terms, and Centenary of Ancient Terms (1849) by Matthew Holbeche Bloxam (1805 – 1888).  Bloxam worked professionally as a lawyer in his hometown of Rugby, England, and was widely known for his archaeological research and writing, particularly on Gothic architecture.  (An obituary appeared in the Journal of the British Archaeological Association, Volume 44, 1888.)  In The Principles of Gothic Ecclesiastical Architecture, Bloxam illustrates several variations of the trefoil arch.  One of his illustrations appears below.

The second arch in the second row and two arches in the bottom row are identified as variations of trefoil arches.

(Bloxam also figures in the historical scrum over the origins of the sport of rugby, but going there would be too much of a digression.)

With that, I bring this cautionary tale to a close, though with one small coda.  I now find myself awash in arches, acutely aware of what had previously remained mostly hidden in the background.  I play with terms such as trefoil and ogee arches, blind and containing arches, intrados and extrados, Gothic and Romanesque.  Trefoil arches make appearances in unexpected places such as New York City's Central Park with its Trefoil Arch.  Perhaps a visit to a natural history museum would help with my recovery, as long as I ignore the architecture of the building.