221 



raised triangular processes, between each two of which is placed a 

 pentagonal plate, about a quarter of an inch in thickness. 



The small one, represented Plate XVII. Fig. 5, exactly agrees, in 

 every respect, with a fossil which is in the British Museum, in the same 

 box with the one already described in the words of Mr. Lister and 

 of Mr. Beaumont. Like that fossil, this smaller one has its trochital 

 articulating surface, its five arms, and its surface made up of polygonal 

 plates ; but it has no opening on the upper surface. On this part, 

 instead of the opening which is observable in Mr. Beaumont's fossil, 

 there are six round protuberances of different sizes ; one projecting 

 about an eighth, and the other about the thirty-second of an inch. 

 The articulating surface in this fossil is very indistinctly seen : it ap- 

 pears to be formed of three pieces, the form of which cannot be po ? 

 sitively ascertained. Around these are disposed six hexagonal pieces; 

 on which other smaller ones are placed, forming the visceral ca- 

 vity ; but the form and arrangement of these cannot be well made 

 out. 



The fossil fragment, Plate ..XVII. Fig. 1, is particularly interesting 

 on several accounts. The crenated articulating surface is very dis- 

 tinctly observable on it, especially towards the edge of the circle. It 

 is also distinctly perceived, that this articulating surface is formed by 

 the union of the flat surfaces of three hexagonal ossicula?, the com- 

 missures of which are not, however, discoverable on the articulating 

 surface, in consequence, as it appears to me, of a portion of the in- 

 tervening cartilage still remaining adherent 011 the articulating sur- 

 face. 



The ossiculae in this fossil, having a more rounded external surface, 

 appear more separated than those in the preceding specimens, and 

 are therefore more distinctly seen : their edges, when viewed with the 

 assistance of a lens, display an osseous structure, very much resem- 

 bling that which is seen on the edges of the fossil bony palates of 

 fishes. 



