106 GASTROPODA. [Aspidobranchia. 



The Trochidce are like the Turbinidce in the possession of a nacreous, 

 test and in the principal structural characters of the animal. They 

 differ from that family in having a corneous, never calcareous, oper- 

 culum, which is always multispiral. 



The animals are herbivorous. The family is represented by 

 numerous species living in the littoral and laminarian zones, and 

 many are deep-sea forms. 



Ordovician to Recent. 



The best works on the Trochidce are Dr. P. Fischer, Monogr. 

 Genre Troque, in Kiener's " Coquilles Vivantes," Paris, 1880 ; H. A. 

 Pilsbry, Tryon's Man. Conch. (1), vol. xi, 1889. 



Subfam. 1. TROCHININ.E. 



Animal with frontal lobes ; jaws wanting ; with 5 lateral teeth 

 on the radula, a sixth obsolete tooth may be present. Shell pearly 

 peristome disconnected. 



KEY TO GENERA. 



1. Shell conical, base flattened ; spire elevated ; last whorl 



generally carinated ; imperforate or false-umbilicate . . TROCHUS. 



2. Shell turbinate, base convex ; generally imperforate ; last 



whorl mostly convex . . . . . . . . MONODONTA. 



3. Shell ovate-conic or pyramidal ; imperforate ; aperture less 



than half the length of shell . . . . . . CANTHARIDUS. 



4. Shell small, conoidal. imperforate or perforate, fine spiral 



lirse ; inner lip callous ; last whorl subangled . . . . PHOTINULA. 



Genus 1. TROCHUS, Linnaeus, 1758. 



Trochus. L.. Syst. Nat., ed. x. 756. Type : T. Niloticiis, L. Pyramided, 

 Swainson. Malacology. 1840, 350. RocJiia. Grav, Guide Syst. Distr. 

 Moll. Brit. Mus.. 1857, 148. 



Animal with 1 to 4 pairs of cirri upon the epipodial line, which 

 are not ciliated ; frontal lobes between the tentacles simple or foliated, 

 distinct or united into a veil. The fornmla of teeth is oo 5 1 5 oo 

 orl + 5.1.54-l oo. The central and lateral teeth have more or 

 less expanded lateral margins, and bear cusps which are smooth on 

 the cutting-edge, with minute denticles at the sides. A narrow oblong 

 plate, lying between the fifth lateral and the inner marginal tooth, 

 is sometimes present. The central tooth is always widest in the 

 middle, tapering toward the cup and the base. 



Shell conical, strong, imperforate or false-umbilicated, the axis 

 always solid, the spire more or less elevated, whorls numerous, gene- 

 rally carinated at the periphery, sometimes rounded, more or less 

 flattened beneath ; aperture rhomboidal. very oblique, columella 

 twisted, its edge generally folded or dentate and toothed at base ; 

 sculptured usually with spiral beaded ridges. 



Distribution. In all the warmer seas. 



