﻿ATHYRIS. 



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Obs. The true character of this species has not always been clearly understood, so 

 that some confusion has resulted from incorrect identifications. Professor Phillips states 

 that his shell is pentahedral, depressed, with the middle of each valve planosulcate • but 

 although some specimens do agree with this description, as well as with the single illus- 

 tration (fig. 2) published in the ' Geology of Yorkshire/ still the larger number are more 

 or less circular or ovate marginally, with their valves equally deep and evenly convex (figs. 

 4, 8 — 10). Professor Phillips was not aware that in the perfect condition his shell was 

 provided with numerous flat concentric plates, which were produced from each successive 

 line of growth, and prolonged, in some specimens, nearly an inch from the surface of the 

 shell (fig. 8), and it is to similar examples that Professor M'Coy applied the generic 

 and specific demonstration of Adinoconchus paradoxus in 1844, and Athyris paradoxa in 

 1855. 



In 1840 M. de Verneuil published a figure of the same shell, with its marginal 

 expansions, which he had found at Vise, in Belgium, under the name of Terebratula de 

 Moyssii} from not being then aware of the difference in character of the expansions in 

 Phillips' and L'Eveille's species. 



These appendages in A. planosulcata have been described as continuous concentric 

 plates, but those in A. de Boyssii are in the shape of numerous concentric ridges, from each 

 of which radiate closely-set fringes of elongated, somewhat flattened, spines. 



In 1843 Professor de Koninck published a very good description of A. planosidcata, 

 along with representations of the shell with and without its expansions. 



It is certain, also, that Atrypa oblonga, of Sow., and Atrypa obtusa, of M'Coy, 

 are only slight modifications in shape of Phillips's species, and in the work on ' British 

 Palaeozoic Fossils' (p. 436), Professor M'Coy still considers A. planosulcata as distinct from 

 his A. paradoxa, but with which opinion I am obliged to dissent. The author, moreover, 

 observes that, " When the extended, flattened lamella? are broken off, as is the case in the 

 greater number of specimens, they only leave traces of obscure lines of growth (about ten 

 in two lines), so nearly obsolete in the rostral portion that it seems smooth, but showing 

 by their thickness, and the extreme obtuseness of the edge in specimens approaching ten 

 lines in length, that to be the ordinary adult size, although I have seen some rather 

 larger not bearing such marks of age. It is only close to, or immediately at, the margin 

 that we find these paradoxical, greatly extended, shelly, flat, radiated lamellae, which, if 

 perfect, would considerably exceed the length of the shell in width. I imagine that they 

 are found at the margins, because there the corresponding lamellae of the two valves would 

 come in contact and support each other ; but, by growth of the shell, they would become 

 separated when a new edge was formed between them, leaving them erect, insulated, and 

 liable to be broken off by the slightest accident. The same thing occurs in Tridacna 

 squamosa, where the great scale-like laminae near the beak are always effaced, while those 

 near the margin are perfect." 



1 'Bulletin Soc. Geol. de France,' vol. xi, pi. hi, fig. 1. 



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