GASTEROPODA. 



33 



By far the most important genus of this family, pala?on- 

 tologically speaking, is that to which the names of Capulus, 

 Pileopsis, Acroculia, and Plcdyceras have been applied. Of 

 these names, the last has been, and is still, very widely used 

 by American palaeontologists, but no valid distinction has yet 

 been pointed out between this and Capulus, and as the latter 

 has the priority over all the titles above mentioned, it will 

 be employed here. In the genus Capulus the shell is conical, 

 with a posterior sub -spiral apex, and a horse-shoe-shaped 

 muscular impression. The 

 aperture is greatly enlarged, 

 and its margins are essen- 

 tially entire ; but owing to 

 the fact that the shell is us- 

 ually affixed for lengthened 

 periods to foreign bodies, the 

 lips may become more or 

 less sinuated or undulated 

 (figs. 420, 421). The shells 

 of this genus may, as a rule, 

 be recognised by their ob- 

 liquely - spiral or straight 



conical shape, their wide aperture, and the absence of a 

 columella. They may be dextral or sinistral, and the sur- 

 face may be simply marked with concentric lines of growth, 

 or may be ornamented with spines. 

 The genus is abundantly repre- 

 sented in the Silurian and De- 

 vonian periods, and less abun- 

 dantly in the Carboniferous. Many 

 Secondary and Tertiary species are 

 known, and the comparatively few 

 living species are widely distributed 

 over the globe. 



Fam. 19. Patellid.e. — Shell 

 conical, with the apex turned for- 

 wards ; muscular impression horse - shoe - shaped, open in 

 front. Foot as large as the margin of the mantle. Eespira- 

 tory organ in the form of one or two branchial plumes, lodged 

 VOL. II. C 



Fig. 420.— Ca}3i(ii(S (Platyceras) ventricosiis. 

 Upper Silurian. (After Hall.) 



Fig. 421. — Different views of Capu- 

 his (Platycems) chimosns, of the nat- 

 tu-al size. Devonian, Canada. (Ori- 

 ginal.) 



