150 DEVONIAN FAUNA. 



species in a rather more advanced stage of growth. It may be compared with 

 PI. XVII, fig. 15. 



With regard to these shells, however, it must be noted that it seems difficult 

 to correlate their internal arrangements with those of Athyris. They agree in the 

 small opening of the visceral canal in front of the muscle-scars ; but there appear 

 differences in the absence of denned ovoid dental sockets, in the strength of the 

 median septum, and in the hinge-plate being apparently produced and curving 

 forward into the beak. Possibly these structures may be due to the immaturity 

 of the shell. They perhaps agree best with those of the sub-genus Semimda, 

 as limited by Hall and Clarke, 1 but according to them that group has a 

 smooth exterior, while judging from the surface the present shell would fall 

 within the sub-genus Cleiothyris as denned by them. 2 



(2) There are also frequently found, though almost always in an extremely 

 crushed and distorted condition, much larger shells, which I formerly believed 

 to belong to a distinct species. These show a broad flattened fold, and a 

 surface of imbricated concentric lines, which appear very similar to those of 

 the smaller form in the central parts, but which become closer and very definitely 

 fringed and spinous marginally (PI. XX, figs. 1 and 2). In one slab of these 

 moulds in the Barnstaple Athengeum is a cast (the mould of which definitely 

 shows the same surface-ornament) which is identically similar to the cast figured 

 by Davidson as " Terebratula elongata, Schlotheim ?," and therefore that name 

 must be removed from our Devonian lists, as in them it is evidently only synonymous 

 with that of the present shell. This cast bears much resemblance to the cast of 

 Athyris lamellosa figured by Hall and Clarke. 3 



The question now arises whether this Devonshire species is identical with any 

 previously described form. M'Coy unites it with A. concentrica, von Buch, and in 

 this he is in part followed by Davidson. The character of the surface, however, 

 seems clearly to negative this. Very indistinct radiations on the ridges of 

 A. concentrica are certainly mentioned by von Buch * and M'Coy, but do not seem 

 to break their continuity; and M'Coy, when identifying Sowerby's shell with it, 

 asserts that " the decussation, to which Sowerby alluded, is only produced by the 

 decomposition of certain of the concentric Iamina3, according with their original 

 fibrous texture, and is not visible in sound specimens." But the evidence of our 

 larger specimens distinctly disproves this assertion. In several of them the rows 

 of puncta on the ridges (as seen in the mould) are much too definite to be explained 

 in any such way ; while in one or two (PI. XX, fig. 1), where the surface is 



1 1894, Hall and Clarke, < Pal. N. Y.,' vol. viii, pt. 2, p. 93. 



2 Ibid., p. 90. 



3 Ibid., pi. xlvi, fig. 20. 



4 1834, von Bucb, ' tlber Terebrat., 1 p. 103. 



