164 On Cyrtoma, a new genus of Fossil Schinida. 



means, we presume, no more than what Professor Gran 

 states in his outlines of comparative anatomy, ' tnat t 

 plates are firmly joined or locked together by thi \ c ~ 

 entrant angles or sutures," particularly as Dr. Sharpey 

 alludes to Echinus esculentus, in which Dr. Grant is 

 of opinion the plates never anchylose. Professor Agassiz, 

 the most recent writer on the subject, in an excellent 

 paper in the Memoirs of the Society of Natural Sciences, 

 Neufchatel, observes, " that in general the testa of the 

 Echini is not so immoveable as one who had not 

 observed them in a fresh state might be led to suppose. 

 All the plates forming the upper part of the disc are often 

 set hi motion, sometimes they sink, sometimes they rise, 

 and, in oblong species the longitudinal diameter is often 

 extended beyond its ordinary length" As I have never had 

 an opportunity of examining these animals in a fresh state, 

 my observations will be of less value ; yet if the motions 

 alluded to by Professor Agassiz depend on a moveable 

 articulation between the various plates, it is difficult to con- 

 ceive how all traces of such articulations should be ob- 

 literated in the dead shell, and how after death different 

 bony pieces should become soldered together, which during 

 life remained separate and distinct from each other. The 

 removal of gelatine and animal matter by bleaching would 

 undoubtedly render the shell generally harder and less 

 yielding than in a living state, but this would not account 

 for the consolidation of all the different plates of which it is 

 composed, if in a living state they were separate and capable 

 of being depressed and elevated at the will of the animal. 

 Since it is those plates only forming the upper part 

 of the disc that are said by Professor Agassiz to be 

 capable of being elevated and depressed, may we not 

 infer from this circumstance that had the individuals ex- 

 amined by him been older than they were, such movements 



