REPORT ON THE ECHINOIDEA. 79 



analogy of the plates of the young Starfish to those of the Crinoids, and which he has 

 most suggestively extended to the Echinids, and which most naturally explains the great 

 number of plates we find both on the ambulacral and interambulacral plates of the 

 Palseechinidse, in which as in the Crinoids the plates composing the calyx subdivide into 

 numerous joints or into numerous plates to form the test of our oldest known Sea-urchins, 

 which- thus still show most unmistakably their systematic afiinity to the Crinoids. 



Unfortunately, in the types with thin coronal plates {Eociclaris, ArchcBocidaris, &c.), 

 owing to the loose cuticle by which these plates were probably connected, much 

 as we find them in the modern Echinothurida3, it is not probable that we shall find whole 

 tests, as even in the recent types when they are dried the plates readily become dis- 

 connected, and we can form no idea of their shape when alive even from well-preser\'ed 

 alcoholic specimens. The abactinal system especially of these genera will very rarely be 

 well preserved, and we can only from analogy w4th the recent types form an idea of the 

 principal structural features of that part of the test. The plates of the actinal system 

 generally hold together more firmly, and from the similarity of its structure in such 

 genera as Archceocidaris, Pholidocidaris, and Lepidesthes, we can fairly assume that the 

 abactinal system corresponds in its general features with that of the Echinothuridse. As 

 far as I can judge from the specimens of PalseechinidEe in my possession which have 

 retained any part of the. actinal region of the test adjoining the actinostome, we find 

 that in Lepidocentrus there is no distinct line of division separating the coronal inter- 

 ambulacral plates from those belonging to the actinal membrane. The ambulacral 

 plates are continued in a remarkably well-preserved specimen from the lower Bur- 

 lington Limestone to the very teeth, and the same is the case with the interambulacral 

 plates ; neither the ambulacral nor the interambulacral plates show any line of demar- 

 cation such as we still find in the Cidaridae or such an indistinct one as exists in the 

 recent Echinothuridse, and as far as I could see in the specimen referred to, the test of 

 this genus was evidently composed of entirely similar plates, extending from the edge of 

 the actinostome, from the very membrane which was attached to the teeth to the 

 abactinal system, that is, the coronal plates extended from the actinostome without the 

 usual subdivision of the actinal membrane into ambulacral and interambulacral plates 

 which in this genus at least did not exist. In fact this genus corresponds exactly to a 

 stage of the Cidaridte in which the coronal plates as they are developed in that family 

 should be reduced to a minimum and replaced by the extension over the whole test of 

 imbricating plates, such as still exist prominently developed in the Cidaridse, and to a 

 more limited extent in some other Echinids, on the actinal membrane. In the Cidaridae 

 proper the junction of these plates with the coronal plates is still quite well marked, 

 while in the recent Echinothuridse the distinction between them is much less apparent. 

 As regards the actinal system of Melonites, from what we know of its structure from 

 Meek and Worthen, it apparently belonged to the same type as that of Lepidocentrus. 



