MALACOZOA. GASTEROPODA. PECTINIBRANCHIATA. 325 



to alter my opinion as to the identity of Trochus cinerarius and 

 Trochus umbilicatus. The latter is always more depressed, 

 larger, with only half the number of grooves on its last turn, 

 and with the radiating undulated stripes or bands much more 

 widely set and broader. 



Common about Slains, Peterhead, and Banff; rarer on the 

 Kincardineshire coast. 



Trochus crebris striis fuscis. Lister, Airim. Angl. PI. 3. f. 15. 

 Trochus umbilicatus. Mont. Test. Brit. 286. Trochus umbilica- 

 tus. Flem. Brit. Anim. 322. 



GENUS 2. MOKODONTA. After Trochus. P. 53, 131. 



Shell imperforate, conoid, or subovato-conical, with 

 the spire moderately elevated or low ; the last turn very 

 large, convex ; the aperture roundish, the peristome in- 

 complete behind, the outer lip thin-edged, the columella 

 sub truncate, or ending in a dentiform prominence. 

 Moi/off, one ; 68ovs, tooth. Lamarck. 



As Lamarck observes, Monodonta is intermediate be- 

 tween Trochus and Littorina or Turbo, the aperture dif- 

 fering from that of Trochus in being more rounded, and 

 from that of Littorina in having the columella truncated . 



1 . Monoddnta crdssa. Solid Monodonta. 



Shell conical, with the spire shorter than the last turn, 

 which is very convex ; the colour whitish, densely un- 

 dulated with dusky lines, the mouth, and part of the 

 lower surface white. Crdssus, thick. 



Shell conical, thick; with the spire much shorter than the 

 last turn, convex in outline, and pointed ; the turns five, the 

 upper little convex, the last very large, convex, very faintly 

 angulate, all obliquely striate ; the suture small, but distinct ; 

 the base convex, the umbilicus covered; the aperture oblique, 

 roundish, with the outer lip thin, the inner incomplete behind, 

 reflexed on the iimbilicus, with the columella straight, at the 

 end truncato-dentiform ; the apex decorticated ; the colour 

 greyish or reddish-white, closely undulato-reticulated with 

 very numerous slender acutely flexuous purplish-black lines ; 



