162 



cies, the central opening is in a smooth concave surface, which is sur- 

 rounded by a finely crenated narrow margin, as is shewn in Fig. 21. 



In the third species, the opening is in the centre of a circular, de- 

 pressed, central surface, which is surrounded by a broad, raised, and 

 finely crenated margin, as delineated in Fig. 22. No representations 

 are given of the entrochi which are formed by these three species of 

 trochitse, since, as their margins are similar, no difference can be ob- 

 servable in their articulations, when viewed externally : for similar 

 4'easons the figures of several of the entrochi are omitted. 



The fourth species, Fig. 23, is of a very curious structure. Its five- 

 rayed opening is surrounded by a small finely striated circular area, 

 around which is a smooth circular groove, surrounded by a raised 

 marginal ridge, the upper and inner part of which is finely crenated. 



The corresponding trochites with the last, appears to be the one re- 

 presented, Fig. 25, the body of which bulges out considerably, and 

 its articulating surface, which is formed of a smooth area round 

 the central opening, surrounded by a margin crenated on its outer 

 part, is exactly fitted to be received into that of the former trochites, 

 and to constitute a series, such as is represented Fig. 44. 



The most common form of the stellated opening is that with five 

 rays. There are deviations, however, from this form, as in the tro- 

 chites, Fig. 26, where the central foramen has three obtuse rays, from 

 which proceed minute stria?, which pass directly to the margin. 



By different arrangements of trochitae, varying like those repre- 

 sented Fig. 23 and 25, in their size and form, entrochi of different 

 figures are obtained, as are instanced in Figures 5, 6, 8, 42, 43 and 44. 

 ~The specimen, Fig. 8, which is a little bent towards its upper part, 

 is particularly interesting, since it serves to demonstrate, that this 

 peculiarity of form is particularly fitted for those entrochi which 

 form that part of the trunk of the animal which is nearest to its head. 

 But this circumstance will be particularly attended to, when inquir- 

 ing into the uses of these parts. 



