290 Mr. H. Seeley on Cambridge Palaontology : — 



Trochus {Trochodon) cancellatus. PI. XI. fig. 15. 

 Shell thin (?), conical, elongated, not umbilicated, commonly 

 composed of four or five slightly convex vi'horls. Each whorl 

 is about twice as wide as high, and ornamented with five or 

 more spiral striae, which are moderately elevated, and have 

 wide spaces between ; the lowest of them is very prominent. 

 These are crossed by numerous close, rather oblique, longitu- 

 dinal striae (about forty on a whorl), which terminate in as 

 many little tubercles crenating the lowest rib of the whorl. 

 The base, which is somewhat convex, is separated from the 

 upper part of the shell by a narrow, flat, oblique band, bor- 

 dered by the bottom striation of the whorl, and a prominent 

 one surrounding the base. The base is marked with about 

 the same number of strise as the whorl ; they are finely gra- 

 nulated. Mouth quadrilateral. At a cessation of growth, 

 little teeth are formed on the inside, which correspond with 

 the external spiral ribs. The columella is banded with two 

 prominent teeth. 



Most specimens have varices, or else internal thickenings of 

 the lip formed at irregular intervals; there are sometimes as 

 many as four on a whorl, and sometimes but one on a shell. 



This is the most variable Gasteropod our Greensand contains. 

 In the casts the whorls are sometimes so rounded as to appear 

 like the cast of a Turho ; sometimes they are perfectly flat, and 

 have the upper one projecting over that below. The spiral 

 angle is not constant. In the casts the longitudinal markings 

 on the whorls are sometimes so faint as to be scarcely perceptible. 



This species is most nearly related to T. Guyotianus (P.& R,), 

 but difi^ers from it in having a cancellated shell and less elevated 

 whorls. 



Not uncommon. University and all other Collections. 



Trochus {Gibbula) levistriatus. PI. XI. fig. 16. 

 Shell elevated, somewhat wider than high, formed of four whorls 

 which regularly enlarge. Whorls angular, with flat sides, a 

 flat space above, and a flat base. The shell is noticeably nar- 

 rower across the upper angle of the whorl than across the 

 lower. The lower is the line of suture. The space on the 

 top of each whorl is nai-row, and, excepting a single central 

 ■ striation which is scarcely perceptible, quite smooth. The 

 side has six striae, including that on each of the two angles ; 

 these are narrow, with wide spaces between. The base has 

 (in the only perfect specimen I have seen) nine striae, which 

 are not similar : on its outer part are three prominent striae, 



