Some new Upper Greensand Gasteropods. 289 



tinuous. Under part of the aperture retreating, as in Pkorus. 

 Base rugosely laminated. Umbilicus open. 



This genus is exactly intermediate between Turbo and Impe- 

 ratar. 



Turhdidea nodosa. PI. XI. fig. 14. 



Shell considerably wider than high, composed of four whorls, 

 which regularly increase in size, and are rather inflated ; they 

 are flattened on the upper part, obliquely sloping, and (but 

 for the lines of growth) smooth. Below the middle of the 

 whorl is a row of large prominent tubercles, which on the last 

 whorl are eight in number ; each of them produces a slight 

 inflation of the shell up to the suture, so that there are low 

 obtuse longitudinal ridges separating wide intervening spaces. 

 Below the tubercles is a narrow space, and then the base. 

 The base is convex, and marked with curved folds radiating 

 from the umbilicus. These rugose folds appear to be produced 

 by thin oblique laminse. Upper part of the mouth nearly 

 straight, and parallel to a plane passing through the axis; 

 but on the base it retreats in a curve. Aperture somewhat 

 oval. Umbilicus longitudinally ridged, of moderate size. 



The cast is smooth, round, and has but slight indications of 

 the tubercles *. 



A specimen, with a portion of the shell preserved and con- 

 verted into phosphate of lime, is contained in the cabinet of 

 Mr. Carter. 



A second species occurs which is nearly twice as wide as high. 

 The whorls are rather narrow, and the last is greatly expanded. 

 Its upper part is a nearly flat ledge. The tubercles (about seven) 

 are very large, and are placed in the middle of the side, or rather 

 above. Below the tubercles the mouth retreats, as in T. nodosa. 

 I only know this species from casts. Its shell was probably 

 thin, and had the tubercles prolonged into spines. It might be 

 named T. expansa. 



Trochodon, n. subg. 



Shell conical, imperforate. Aperture quadrate. Columella 



toothed. 



This little group of shells will perhaps be best placed as a 

 subgenus of Trochus, to which genus the species are at present 

 referred. 



* Turbo Delafossii (D'Arch.) is a nodulated shell, but in no other way 

 resembles this, diflFering generically. 



