164 



have evidently been intended for the purpose of articulation ; since, 

 on two of these trochita?, a real crenated articulating surface is ob- 

 servable. 



Fig. 28 represents a single trochites, on one side of which is a cir- 

 cular, concave, articulating surface, with a crenated edge and a cen- 

 tral foramen. 



Entrochi are sometimes formed of trochita?, which are smaller at 

 one end than at the other ; and the trochitae being adapted to each 

 other, by the small end of the larger being attached to the larger ei\d 

 of the next trochites in size, the entrochus itself bears a conical form. 

 Other varieties are observable in the forms, both of trochitae and of en- 

 trochi, which will be noticed as they occur. 



There is, however, one species of trochites which may be noticed in 

 this place. This species is chiefly found, it appears, in Somersetshire: 

 it is of an oval form, with a central foramen, from which, towards 

 each end of the trochites, passes a slight ridge, and in others a small 

 groove. The following account of this fossil, given by Mr. Beau- 

 mont, jun. of Stony Easton, in Somersetshire, appears to be so 

 very accurate as to claim being introduced here in his own words*. 



" Mr. Lister mentions one trochite he found, of an oval figure, the 

 rays scarcely apparent, and a very small point in the place of the pith. 

 I have of this species, with entrochi the same, (if these, having lost 

 the figure, may retain the name of T(0%*) some of them have good 

 large holes in the middle, like other trochites ; but their bore is oval, 

 according to the stone. I have many other trochites of this kihd, 

 but with this difference, that these have no rays, but are joined toge- 

 ther only by one ridge, which passes directly along the middle of the 

 stone, the long way, there being a furrow in the other answering to it ; 

 these have also a small peck in the middle, making but very, little 

 impression in the stone, and seldom passing through it, though 1 have 



* Philos. Transact. Vol. XT. p. 121. 



