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of the trochites are rounded, the sides of the entrochus must necessarily 
be formed in alternate round ridges and depressions. 
The third species, Fig. 15, has its opening in the centre of a de- 
pressed area, the broad-raised margin surrounding which is radiated 
with fine striae, like the former. This trochites is noticed by Rosinus, 
Tab. VII. Cap. III. A. 3. 
The fourth species. Fig. 11, has a flat, smooth, central surface, sur- 
rounded by a crenated margin, and is of a cylindrical form. Tro- 
chitae of this species are figured by Rosinus, Tab. VII. Cap. I. A, 1, 
2. 3, 4; Cap. II. A. 1, 2, 3, 5, 6. The entrochi belonging to this spe- 
cies, Fig. 1 and 2, assume different forms dependent on the shape of 
the sides of the trochitae, of which they are composed ; the trochitae 
in the one. Fig. 1, being gently depressed in the middle part ; whilst 
in the other. Fig. 2, they possess a small degree of convexity ; the 
articulating surface in each specimen being formed by a smooth cen- 
tral surface, with a crenated margin. Fig. 12 represents one of this 
species, which has a smooth central surface surrounded by a deeply 
crenated margin ; the radiating eminences being so broad and so 
rounded off, as to bear somewhat of the appearance of a folded leaf. 
Rosinus describes several trochitae which have a similar appearance. 
The corresponding entrochus, as might be expected from the size of 
the marginal notches, is characterized by the greater width and round- 
ness of the identations and markings at the articulation. Rosinus 
describes another trochites, in which a circle is interposed between the 
central hole and the crenated margin. He also describes another tro- 
chites of a singular kind, having the two sides unequally marked; 
eleven rays being observable on the margin of one side, and fifteen on 
that of the other. The trochites. Fig. 17? which I consider as the 
fifth species, has rays of a curious structure. Concentric depressed 
circles, closely set, repeatedly divide each ray into several parts. 
The surfaces, therefore, are covered with small protuberances, regu- 
