164 
have evidently been intended for the purpose of articulation ; since, 
on two of these trochitae, a real crenated articulating surface is ob- 
servable. 
Fig. 28 represents a single trochites, on one side of which is a 
circular, concave, articulating surface, with a crenated edge and a 
central foramen. 
Entrochi are sometimes formed of trochitae, 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 end 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 entrochi, 
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. Beaumont, 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 Tpox (^ ;) 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 kind, 
but with this difference, that these have no rays, but are joined 
together 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 I have 
* Philos. Transact. Vol XI. P.727. 
