Notes on Spondylus excavatus Chenu,1845
By Michele Dardano

After several years of research, not without difficulty, given the scarcity of material available, I managed to put together some Spondylus from the islands of Cabo Verde. The news that accompanied the description of Spondylus excavatus since its discovery ( 1845) were very scarce and contradictory, so I found it useful to make a new description, trying to use all the news about this species, accompanied by some of my personal observations.

I'll try to point out the most controversial aspects of previous discussions and, above all, try to show other members of this elusive species, differentiating it from other similar species, i.e.
Spondylus senegalensis Schreibers, 1793 and Spondylus gaederopus Lynnaeus, 1758.I want to point out that my clarifications are derived from direct observation of whole specimens,and that the place of origin is undoubtedly reliable. Most specimens,in fact, come from the island of St. Vicente.  It's well known to collectors and scholars of this family that it is extremely risky to establish a diagnosis on a single specimen,or even, on a single valve, especially when it is abeached specimen.

It's necessary to have, as far as possible, several samples of the same (and similar) species  in order to make appropriate comparisons.

             STRUCTURE: very irregular, excavated surface (hence the name), oblique and inequivalve: the lower valve is deeper, sometimes dramatically.

             SCULPTURE: 11-12 main radial ribs bearing small nodules or short, depressed spines. Sometimes the nodules are absent altogether, sometimes the spines are slightly longer at the margins.  The interstices show numerous fine riblets, visible especially near the umbonal area; on the rest of the shell, they become very irregular, so as to make their individuation difficult. Anyway, the ribs are crossed by concentric striae that give the  interstices a nodulose appearance, which, on the other hand, is not always visibile.

              Another important characteristic, visible in all the specimens available to me, is the presence of a strong and dense denticulation at the hinge area. This denticulation is also visible in the holotype, housed in the Museum of Geneva.The auricles are quite large and well spaced, another important feature, in my opinion, in order to differentiate this species from other similar species.  I calculated the ratio between the shell height  and the distance between the auricles in 4 specimens of
Spondylus from Cape Verde and 4 specimens of Spondylus senegalensis (2 of them belonging to the variety Spondylus canariensis, 1 to the variety powelli and 1 to the variety senegalensis senegalensis).

Two of these  are figured in Lamprell's book.  The specimens of Cape Verde have given, as the ratio, the following values: 1.57, 1.44, 1.66 and 1.53 (mean value: 1.55), while specimens of
Spondylus senegalensis gave, as the ratio, the following results: 2.07, 2.12,  2.18, and 2.0 (mean value: 2.09).

COLOR: contrary to what was stated by Lamprell in the posthumous book "Spiny Oysters", the color is purple-violet on both valves.  The color of the left valve is significantly darker than the right valve. The same color is observed, although slightly faded,in the holotype, due to the fact that it dates in the mid 1800.    Internally, color is blue-white with a dark purple or violet margin, most evident in the left valve
.Often, the hinge teeth are stained  with brown.

                SIZE: up to 80 or 90 mm.The largest specimen,that I have in my collection, measures 110 mm.  Strangely enough,
Spondylus excavatus has been described as a small species, in the order of 35 mm, but this is evidently an error, because the holotype, in the Geneva Museum, measures 65-70 mm.

                ATTACHMENT AREA: variable, but not very large.

REMARKS: I compared the photos of the specimens in my possession, especially

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