90 DEVONIAN FAUNA. 



This fossil does not appear to agree with any of the other Cephalopods from 

 these beds. The nodes are larger and more numerous than in Tr. Vicarii, and 

 the dimensions are different from that shell and from Tr. pulcherrimu7n. It is 

 almost exactly like the Ch/roceras, sp., figured by Sandberger (pi. xiii, fig. 3), 

 though the back is more arched, and consequently the section is more rounded. 

 At first sight I was inclined to refer it to Orthoceras arcuatum, Steininger,^ as 

 described from South Petherwyn by Phillips under the name of Cyrtoceras 

 Tusticum,^ but this resemblance is only due to the two fossils being in a similar 

 state of preservation. The chambers in the latter shell are much closer, the 

 nodes fewer, and the siphuncle marginal. In fact, it belongs not only to a 

 different species but to a different genus. A comparison with one of the 

 specimens of Tr. pulcherrimuni (fig. 4 a), which has the under layer of test 

 exposed, enables us to imagine the character of the ornamentation, but the frag- 

 ment is too imperfect to admit of specific description and determination. 



Locality. — Wolborough. 



III. Family. — Cyetoceratid^. 

 1. Genus. — Gyeoceeas, De Eoniftck, 1844. 



This genus is formed for Nautiloid shells whose chambers are bent into a spiral 

 of one or two volutions, which lie in one plane, and are often discontiguous. It is 

 distinguished from Cyrtoceras by the shell forming a distinct spiral and not simply 

 an arch. From Trochoceras it is separated by the spire being in one plane. 

 From Lituites, Nautilus, Hercoceras, &c., it is differentiated by its coils being loose 

 and rapid. Its aperture is simple and sometimes lobed. 



It will be seen in the following descriptions that these generic characters of 

 Oyroceras fail sometimes at least in one respect. We find that many of the 

 species are not actually symmetrical, though the deviation from the plane of 

 symmetry is so slight as to have been unobserved. Such is the case with 

 G. ornatmn (Goldf.) itself ; and this would point to Gyroceras being allied to 

 Trochoceras rather than to Nautilus. A similar slight asymmetry is also to be 

 noticed in certain species of Cyrtoceras. Thus portions of these three genera, 

 Cyrtoceras, Gyroceras, and Trochoceras, are principally divided by the amount of 

 curvature of the spire, a feature which is of little anatomical value. 



Gyroceras is represented in the Silurian, Devonian, and Carboniferous 

 formations. 



1 1834, Steininger, ' Mem. Soc. Geol. Fr.,' vol. i, pt. 2, p. 396, pi. xxii, fig. 6. 



2 1841, Phillips, ' Pal. Foss.,' p. 116, pi. xlvi, fig. 222. 



