PLATE 57. 
Page 
Fig. 1 a, h. Platyceras billingsi. 315 
1 a. View of the aperture of this species. [ The striae on the lower side of the figure are very 
incorrectly represented. ] 
1 b. View looking upon the spire of the same specimen, the upper part of which shows the 
suhangular form and the sinuosity of the striae. 
Fig. 1 c. Stropiiostylus? rotundatus. 307 
1 c. View of the spire, with the aperture downwards. 
Fig. 2. Platyceras sinuatum. 314 
2. View of the spire of a specimen of this species. 
Fig. 3. Platyostoma arenosa. 302 
3 a. View looking upon the spire. 
3 b. View of the aperture of the same. 
Fig. 4. Platyceras ventricosum. 311 
4. A large individual which is extremely ventricose. The specimen is silicified, and the surface 
striae are obliterated. 
Fig. 5. Platycjeras trilobatum. 310 
5 a. View of the aperture and part of the spire of a large individual. [ The striae represented 
between the aperture and the next volution do not exist in the specimen.] 
5 b. The lower or umbilical side of the same specimen, showing the sinus on that side. 
5 c. The upper side of the specimen, showing the sinuosity on that side and the extension of 
the shell in the middle. The upper volutions are imperfect. 
Fig. G. Pleurotomaria labrosa. 339' 
0 a. Profile view, looking upon the back of the specimen. 
6 b. View of the lower side of the aperture, the upper side being broken away. 
