PROSOBRANCHIATA. 



767 



open into the cavity of the shell internally, and externally by minute 

 pores on the margin of the wing. Again, in the Triassic and Jurassic 

 genus Trochotoma (fig. 651, b) the shell is trochoid, with a concave base, 

 and there is a single elongated perforation in the slit-band, near the 

 margin of the outer lip. The ancient Ordovician types Scalites, Rhaph- 

 istoma, and Helicotoma, of which the two last are probably identical, 



Fig. 651. — A, Scalites angulatus, Ordovician, North America. (After Hall.) 

 B, Trochotoma affinis, Jurassic. 



appear to be really referable to Pleurotomaria. In Scalites (fig. 651, a) 

 the shell is spiral, with a flattened spire, the body-whorl ventricose, the 

 " suture " canaliculated, the lip truncated, and the columella imperforate 

 and curved. In Rhaphistoma 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 slightly 

 notched. 



The genus Porcellia (fig. 652), sometimes regarded as belonging 

 to the Bellerophontidce, seems 

 to be really referable to the 

 Pleurotomariidce. In this 

 genus the shell is discoidal 

 and many-whorled, the outer 

 lip having a deep slit, and 

 the whorls having a well- 

 marked revolving band run- 

 ning along the centre of the 

 dorsal side. The species of 

 Porcellia appear to range 

 from the Devonian to the 

 Trias. 



Lastly, the important genus 

 Murchisonia (fig. 653) is nearly related to Pleuroto?naria, the line 

 of separation between the two groups being, in fact, one which can- 

 not be sharply defined. The shell in the typical forms of Murchi- 

 sonia is long and turreted, the number of the whorls being greater 



Fig. 652. — Porcellia puzo, viewed sideways (a) and 

 from the front (b). Carboniferous. 



