ilALACOZOA. GASTEROPODA. PECTINIBRANCHIATA. 13 1 



ment would be useless, for in many cases the student 

 could not decide whether a mouth should be considered 

 as angular, round, or pyrate ; and all these forms occur 

 even in a single most natural genus, Littorina. 



GENUS 1. TROCHUS. LINN. PYRAMID-SHELL. 



Animal spiral ; head broad, with two long, slender 

 tentacula ; eyes at their outer base ; mouth with a long 

 spirally rolled lingual filament, passing into the abdomen ; 

 foot rather short, thick, rounded at both ends. A thin 

 horny operculum, with numerous narrow spiral turns. 



Shell conical, with the spire moderately elevated, or 

 low, the turns flat or somewhat convex, longitudinally 

 sulcate or striate, the last large, angulate along the 

 middle ; the aperture somewhat square or angulate, ob- 

 liqne, the margin incomplete behind, the columella some- 

 what arcuate and often projecting a little, the outer lip 

 thin ; the inner layers of the shell pearly. 



The Trochi feed on vegetable substances ; adhere to 

 rocks, stones, and other hard bodies, or to plants, and 

 creep in the manner of snails. 



1. Trochus SisypJi'mus. Sisyphine Pyrami&ShelL 



Shell conical, with the pillar closed, the base flattened, the 

 aperture somewhat square, oblique ; the interior pearly and 

 iridescent, the lip very thin ; the last turn angulate ; all the 

 turns, eight or ten in number, flat, glossy, faintly striate trans- 

 versely, marked longitudinally with from three to five grooves 

 and corresponding inconspicuous delicate ridges, of which the 

 upper is larger, and a lower prominent rim with a single 

 groove upon it ; the colour pale reddish-yellow, the raised band 

 on the spire and angle of the last turn with alternate whitish 

 and red spots, the apex often purple ; the base concentrically 

 grooved, and radiatingly striate ; the outline of the spire rather 

 concave, the apex very acute, the ridges of the upper turns 

 granulate. 



I have seen only a few live specimens, of which four are in 

 my collection, brought up by the fishing lines, off Aberdeen. 

 The first was found by Miss Marion Macgillivray. Mr. Alex. 

 Murray has also found it on the coast of Peterhead and St. 



