200 



ON THE TOPOGEArHY AND GEOLOGY 



PART lY. 



PALAEONTOLOGY. 



CHAPTER XIIL 



DESCRIPTIVE CATALOGUE OF THE FOSSIL MOLLUSCA. 



PTEROPODA. , .. 



DIACRIA. Gray. . > ■ 



D, bisulcata. Gabb, n. s. , ■ 



Shell allied to D. trispinosa, Lesueur in general form ; lateral spines broader and shorter ; apex straight, broad, 

 compressed vertically, lateral margins carinate ; body sub-copipressed ; aperture short, upper lip much more curved 

 than the lower, producing a rather gaping mouth ; both lips bordered by a reflected margin. Surface of both vipper 

 and lower sides marked by two long longitudinal depressions, which converge towards the base of the apical spine, 

 producing a broad elevated median portion. 



This shell is less gibbous than D. trispinosa, and more so than D. mecronata. 

 From the former it differs also in the shape, and from the latter in the direction of its 

 lateral spines. The sulcation on the back also leaves the median space wider than in 

 Qnoj and Gaimard's species, and the reflexed lips of the aperture are different from 

 any described species. 



BALANTIUM. Leach. ' - " ' 



B , u n d u I a t u m. Gabb, a. s. 



Shell sub-cylindrical, tapering convexly towards the apex which is slightly arched downwards. Transverse 

 section ellii^tical ; aperture as wide as the shell, but slightly compressed vertically. Under surface broadly undu- 

 lated by a few transvei'se wrinkles ; marked on each side near the apex by a faint marginal groove. 



i^earest to B. australe, d'Orb., as figui-ed in Amer. Merid. but without the lateral 

 riclges. It also differs in being distinctly curved instead of straight. The figure 

 given by Cheuu in his Manual is entirely unlike that of d'Orbigiiy. 



STYLIOLA. Lesueur. ..." , • ■ 



S. sulcifera. Gabb, n. s. . 

 Shell minute, regularly conical, straight ; marked on one side by a longitudinal groove. 



This species can be at once distinguished by its groove. It is not unlike 8. striata, 

 Rong., in the width of its apical angle, but differs from that species in not possessing 

 its curve nor its transverse striations. 



