PROSOBRANCHIATA. 



765 



Fig. 647.— Different views oSCafni- 

 lus {Platyceras) dumosus, of the nat- 

 ural size. Devonian, Canada. (Or- 

 iginal.) 



or may be ornamented with spines. The genus is abundantly 

 represented in the Silurian and Devonian periods, and less abun- 

 dantly in the Carboniferous ; while 

 ancient types have been described from 

 the Upper Cambrian and Ordovician 

 rocks. Many Secondary and Tertiary 

 species are known, and the compara- 

 tively few living species are widely dis- 

 tributed over the globe. 



According to Lindstrom, the name of 

 Platyceras may be retained for the Silu- 

 rian forms of Cabidus, as presenting cer- 

 tain constant differences, and, in particu- 

 lar, as not having been clearly shown to 

 possess the horse-shoe-shaped muscular 



scar of the latter. The same authority unites with Platyceras the Palae- 

 ozoic types described under the names of Platystoma (Conrad) and Stro- 

 fihostylus (Hall). Orthonychia* again, includes forms with a nearly 

 straight shell, the spire being very small, and the terminal portion of 

 the shell very large. 



Family 4. Velutinid.e. — The principal genus in this family is 

 Velutina, in which the shell is thin, with a large body-whorl and 

 small spire, and having a large rounded aperture with an entire 

 margin. The genus is principally Recent and Tertiary, but a few 

 Secondary types have been described. 



Family 5. Pleurotomariid^e. — In this family the shell is nacre- 

 ous, very variable in form, but usually coiled into an elevated or flat 

 spiral. The outer lip of the shell-aperture (fig. 648) exhibits a notch 

 or slit, which in course of growth becomes progressively filled up, 



Fig. 648. — Side-view of the recent Plenrotomaria Quoyana, showing the slit in the outer 

 lip {s) ; and upper view of Pleurotouiaria platyspira, showing the slit-band. 



thus giving rise to a revolving band upon the whorls ; or which may 

 become partially closed and converted into one or more perforations. 

 The animal is provided with a circular horny operculum. 



The family Pleurotomariidce has relationships with the Haliotidce, 

 the Bellerophontida:, and the Euomphalidce ; and the great majority 

 of the genera included in it are extinct. If Scissurella be placed 



