PROSOBRANCHIATA. 



769 



whorls are few, smooth or sculptured, and there is a dorsal band or 

 keel along the convex margin of the shell. The aperture is often 

 more or less expanded, and is in most instances emarginate or 

 deeply notched on the dorsal side, the columellar lip being generally 

 more or less callous. The genus ranges from the Upper Cambrian 

 to the Permian, but attains its maximum in the Carboniferous 

 Limestone. Bucania (Silurian to Carboniferous) includes forms not 

 generically separable from Bellerophon, but distinguished by the 

 fact that all the volutions are visible and increase gradually in size 



Fig. 655. — Bellerophon bicarcnus, from the Carboniferous rocks of Belgium. (After Zittel.) 



to the expanded mouth. The genus Trenumotus (Silurian) resembles 

 Bucania in the form of the shell ; but the dorsal band is replaced 

 by a series of perforations which become successively filled up in 

 process of growth, and which appear to correspond with the siphonal 

 apertures in the shell of Haliotis. Closely allied to T?'emanotus is 

 the Silurian and Devonian genus Salpingostoma, in which there is a 

 single elongated aperture on the dorsal margin of the body-whorl at 

 some distance behind the margin of the lip. 



In the neighbourhood of Bellerophon must also be placed the 

 genus Cyrtolites, in which the shell is symmetrical, discoidal, or 

 coiled into the shape of a horn, the whorls being in 

 contact or more or less disconnected. The aper- 

 ture is rhomboidal, with a median sinus, and a 

 band or keel is developed on the convexity of the 

 last whorl, while the surface is often sculptured. 

 The genus ranges from the Ordovician to the 

 Carboniferous. 



Fig. 656. — Cyrto- 

 lites ornatus. Ordo- 

 vician. 



The affinities of the genus Bellerophina, of the Gault 

 (Cretaceous), are quite uncertain, though it has often 

 been associated with Bellerophon. The shell in this 

 genus is globular, spirally inrolled, and nautiloid, but 

 the outer lip has no sinus or notch, and the convexity 

 of the whorls is not furnished with a keel or band. It is not improbable 

 that this genus is really referable to the Hcteropoda, but the structure 

 of the shell is at present imperfectly known. 



vol. 1. 3 c 



