17G On Cyrtoma, a new genus of Fossil Echinida. 



The Holothuria have been usually stationed near to the 

 Echinida, some naturalists placing them at the head of the 

 Echinodermata, some at the end of the order, and some 

 altogether doubting their connection with it, but none 

 taking the trouble to examine the question ; and the two 

 most recent writers, MM. De Blainville and Agassiz, as 

 if to prove how easy it is to retrograde in science, have 

 placed the Holothuria before the starfishes, although the 

 slightest attention to the philosophical views of Mr. MacLeay 

 on the subject of affinities, should have shown that they 

 were placed very nearly in their true position by Cuvier. 



Before quitting this part of the subject, to which I trust 

 the kind offices of friends in forwarding collections from 

 the coasts will soon enable me to return, I must refer 

 to an interesting analogy which I observed between the 

 Echini and Chelonion reptiles, before I was aware that Mr. 

 MacLeay had noticed similar relations between these groups. 

 On the carapace, or bony plates composing the back of 

 several tortoises, after the upper crust or tortoise shell is 

 removed, we perceive deep lines or grooves marking the 

 boundaries between the plates of the latter, exactly similar 

 to the grooves already described on the testa of the Echini, 

 characterising the five divisions into which the skeletons of 

 aged individuals fall. 



In both cases this appears to be owing to a peculiar 

 function in the external envelope of the skeleton. In the 

 tortoise, moreover, the plates of shell are radiated and 

 raised in the middle, so as to represent spines, and in some, 

 as Testudo serpentina, Lin. and T. caretta, Gm. they are 

 actually accumulated to points. I now however find that 

 guided by the peculiar circumstances of the nervous and 

 osseous systems, as well as the general forms of the two 

 groups, Mr. MacLeay, in the Horas Entomologicee, has made 

 them occupy corresponding places in the respective classes 

 to which they belong. 



Having traced the affinities of Echinodermata sufficiently 



