30 



GASTEROPODA. 



In the Jurassic Ditrcmaria there is a kidney-shaped aperture, 

 consisting of two foramina united by a slit, in the hand near 

 the outer lip. Lastly, in the Jurassic genus Trochotoma, the 

 shell is trochoid, with a concave base, and there is a single 

 elongated perforation in the band near the margin of the 

 outer lip. 



It would seem probable that the ancient Lower Silurian 

 types which have been described under the names of Scalites 

 and Baphistoma should be placed in the neighbourhood of 

 Pleurotomaria. In Scalites (fig. 414, e) the shell is spiral, 

 with a flattened spire, the body- whorl ventricose, the "suture" 

 canaliculated, the lip truncated, and the columella imper- 

 forate and curved. In Raphistoma the spire is still more 

 depressed, the " suture " is close, instead of being grooved, 

 there is an umbilicus of moderate size, and the aperture is 

 somewhat trigonal and sliglitly notched. 



High authorities now also place here the genus Porccllia 

 (fig. 417), and there are certainly strong grounds for accept- 

 ing tliis view, though 

 this type has usually 

 been regarded as refer- 

 able to the Heterojoods, 

 and it has unquestion- 

 able points of resem- 

 blance with the Bcllcr- 

 ophontida:. If we j)ro- 

 visionally place Porccllia 

 in this connection, we 

 have to regard it as a 

 discoidal Pleurotomaria, 

 in which there is a dorsal band, in the median line, with 

 a deep slit in the outer lip. This last feature is undoubtedly 

 one which brings the genus in close connection with Pleuro- 

 tomaria ; and as some forms of the latter are very nearly 

 discoidal, it is perhaps best to remove Porccllia from its 

 association with the Heteropodous genus Bcllcrojplion, in 

 which the outer lip of the aperture is sinuated or notched, 

 but not slit, and there is no dorsal band. So far as known, 

 Porccllia is Devonian, Carboniferous, and Triassic in its range. 



417. — Porcellia puzo, viewed sideways (a) and 

 from tlie front (b). Carboniferous. 



