OF CENTRAL CANADA PART IV. 281 



Holostomata : In this section, the aperture or so-called mouth of 

 the shell presents a continuous or entire margin : hence holosto- 

 matous types are often called " round-mouthed gasteropods," but the 

 same character is presented by the Helicidce, (snails) and other Pul- 

 monata. It indicates, as a rule, vegetable-feeding types. Examples 

 of Holostomata are of common occurrence in all fossiliferous rocks, 

 and they abound on existing sea-coasts ; but whilst they are associ- 

 ated at present and in Cainozoic and Mesozoic strata, with siphon- 

 ostomatous types, in the rocks of the various Palaeozoic periods they 

 appear to form the sole representatives of the prosobranchiate gas- 

 teropods. Carnivorous forms of mollusca were then mainly repre- 

 sented by predatory, coast-frequenting cephalopods, which afterwards 

 became extinct. The holostomous gasteropods of our Silurian and 

 Devonian strata belong chiefly to the following genera : Pleurotomaria : 

 Shell with few whorls ; the lower whorl large, with slit in outer lip. 

 The slit becomes closed with the growth of the shell, but its position 

 is indicated by a more or less distinct band (fig. 219). Murchisonia : 

 Shell spiral or sub-spiral, with many whorls ; a slit or band present, 

 as in pleurotomaria (fig. 220). Bellerophon (formerly referred to the 

 Heteropoda} : Shell boat-shaped, somewhat globular, equilaterally 

 curved; a slit in outer lip often present (fig. 221). Gyrtolites, like 

 Bellerophon, but mostly compressed ; with distinctly-seen whorls and 

 keeled back. Euomphalus (including Ophileta*) : Shell discoidal, 

 more or less regularly enrolled ; with, typically, sunk spire, large 

 umbilicus, and sub-angular whorls and aperture. Maclurea (formerly 

 referred to tha Heteropoda) : Shell with flat spire and rapidly enlarg- 

 ing outer whorl, furnished with a thick operculum, shewing two 

 projections on its inner side (fig. 222). Platyceras ( - Acroculia) : 

 Shell bonnet-shaped or patelliform, with very large aperture (fig. 

 J23). Platyostoma: Shell turbinate, few-whorled, the lower whorl 

 very large, often projected laterally, with aperture wider than high. 

 ffolopea : Shell turbinate, few-whorled ; aperture round, or higher 

 than wide, with continuous margin (flg. - 4). Cyclonema : Shell 

 turbinate, few-whorled, much like Holopea, but with somewhat inter- 

 rupted margin. Subulites : Shell elongated, cylindrical, with com. 

 paratively narrow aperture (fig 225). 



* Ophileta is properly a Euomphalus with slightly raised spire ; or a low-spired Straparol- 

 lus, if Straparollus and Euomphalus be considered distinct. 



