;68 



DIVISIONS OF THE GASTROPODA. 



than in Pleurotomaria. The outer lip is deeply notched, and the 

 outer side of the whorls shows the characteristic slit-band of the 

 family. The aperture of the shell is slightly channelled in front, 

 and the surface is often variously sculptured and 

 adorned. The genus ranges from the Ordovi- 

 cian to the Permian rocks, and possibly is repre- 

 sented in the Alpine Trias. 



Family 6. Bellerophontid^e. — In this family 

 the shell (figs. 654, 655) is " nautiloid, generally 

 symmetrical, spirally inrolled ; the spire is con- 

 cealed by the succeeding whorls ; the aperture is 

 of large size ; the outer lip is arched, sharp-edged, 

 simple, or greatly expanded, carrying in the middle 

 line a sinus or notch, continued on the convex- 

 ity of the last whorl by a slit-band or by a series 

 of perforations" (Fischer). This family includes 

 the extinct genus Bellerophon and its allies, and 

 has been often referred to the order of the Heter- 

 opoda. The shell in Bellerophon is, however, of 

 considerable thickness, and the genus is com- 

 monly found associated with forms which undoubtedly inhabited 

 water of comparatively small depth. For these reasons, amongst 

 others, the Bellerophontidce can hardly be associated with a purely 

 oceanic group such as the Heteropods. On the other hand, the 

 presence of a dorsal revolving band on the whorls, and of a sinus or 

 notch in the outer lip, brings the shell in relation with that of the 



Fig. 65 3 . — Murchi- 

 sonia gracilis. (Hall.) 

 Ordovician. 



Fig. 654. — Bellerophon Argo (Billings), a, Front view; b, Side view. Ordovician. 



Pleurotomariidce. ; while an affinity with the Haliotidce is established 

 by the presence in some forms (Tremanotus) of a row of apertures 

 on the dorsal keel of the shell. All the members of the Bellero- 

 phontidce are extinct, and if the problematical Cretaceous genus 

 Bellerophina be excluded, they are exclusively confined to the 

 Palaeozoic rocks. 



In the genus Bellerophon (figs. 654, 655) the shell is symmetri- 

 cally convoluted, the coils of the shell lying in one plane. The 



