160 



of the trochites are rounded, the sides of the entrochus must neces- 

 sarily be formed in alternate round ridges and depressions. 



The third species, Fig. 16, 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 characterised by the greater width and round- 

 ness of the indentations 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- 



