Hyatt] 298 [April 4, 



Sphyradoceras, 1 nobis, includes a series of Silurian and De- 

 vonian, annulated, costated, and longitudinally ridged species 

 whose close affinity to, and probable derivation from, Spyroceras, 

 will hardly be disputed. Ridges and costae are both present in 

 the young, but in succeeding stages one or the other, or both may 

 disappear. The species are more or less trochoceran in mode of 

 growth, with arcuate and gyroceran forms. 2 They have straighter 

 sutures than in Hercoceras besides the differences of the shell 

 markings, and the position of the siphon varies from near the 

 centre to near the venter. The sutures have either straight sut- 

 ures, or saddles on the venter, and also saddles on the dorsum. 

 In some species there is a lateral line of tubercles similar to those 

 of Heroceras, but smaller ; and the inner lip of the aperture may 

 also occasionally bend upwards as in Hercoceras. Notwithstand- 

 ing these peculiarities the flattened sides and abdomen of adults 

 and their apertures, which are similar to those of Barrandeoceras ; 

 and the compressed elliptical whorls of the young of arcuate spe- 

 cies like Sphy. (Troch.) debile sp. Barr. pi. 18, and their central 

 siphon, leads one to associate this genus in the same general series 

 with Barrandeoceras. This group may possibly help us to explain 

 the presence of the longitudinal ridges and annular costae in 

 Barrandeoceras, when more perfect records are available. They 

 may be at present considered a series of shells with slight trocho- 

 ceran deformation, which is an offshoot of Spyroceras, and pos- 

 sibly nearly related to the ancestral forms of the Nautilidae. 

 Type, Sphy. (Troch.) Clio. sp. Hall, Nat. Hist.K Y., Yol. 5, pt. 2, 

 pi. 59, 111. Mus. Geol. Surv. Albany. 



Uranoceras, nobis, includes arcuate and gyroceran forms of 

 the Silurian with large, stout, elliptical, or laterally compressed 

 whorls, abdomen and dorsum convex and about equal in breadth. 

 Siphon is near but above the centre. The sutures have very 

 broad ventral saddles, slight, lateral lobes, and broad dorsal sad- 

 dles in the early adolescent stages, and acquire very slight ventral 

 and dorsal lobes in the adults of the American species. The si- 

 phon is large and nummuloidal, and the peculiar gibbous whorls, 



™ 24>v'pas. a cake of dung. 



2 One species, Sphy. (Troch.) nodosum, sp. Barr. pi. 20, fig. 20 r appears in only one 

 variety to have had a very faint impressed zone, and may possibly come within our 

 definition of the nautilian shell. 



