358 Prof. A. Agassiz on Palceqntological 



shaped outline of the test. Little by little the test loses with 

 increasing age this Cidaris-Wkz character; it reminds us, from 

 the increase in the number of its plates, more of Hemicidaris, 

 then, with their still greater increase, of the Pseudodiadema- 

 tida3, and, finally, of the Echinometradse proper. The spines, 

 following pari passu the changes of the test, lose little by 

 little their fantastic embryonic or, rather, Cidaris-like appear- 

 ance, and become more solid and shorter, till they finally as- 

 sume the delicately fluted structure characteristic of the Echino- 

 metradffi. The vertical poriferous zone is first changed into a 

 series of connected vertical arcs, which become disjointed, and 

 form, with increasing age, the independent arcs of pores, 

 composed of three or more pair of pores, of the Echino- 

 metradge. 



In the Echinida? proper we find in the young stages the 

 same unbroken vertical line of pores, which gradually becomes 

 changed to the characteristic generic types. We find, as in 

 the Echinometradse, an anal system closed with a single plate, 

 and an abactinal system separating in somewhat more ad- 

 vanced stages from the coronal plates of the test. This is as 

 yet made up of a comparatively small number of plates, carry- 

 ing but few large primary tubercles, with fantastically shaped 

 spines entirely out of proportion to the test, but which, little 

 by little, with the increase of the number of coronal plates, 

 the addition of primary tubercles, and their proportional de- 

 crease in size, assume more and more the structure of the 

 genus to which the young belongs. The original anal plate 

 is gradually lost sight of from the increase in number of the 

 plates covering the anal system, and it is only among the 

 Temnopleurida? that this anal plate remains more or less 

 prominent in the adult. In the Salenidse, of which we know 

 as yet nothing of the development, this embryonic plate re- 

 mains permanently a prominent structural feature of the 

 apical system*. 



Among the Clypeastroids the changes of form they undergo 

 during growth are most instructive. We have in the young 

 Fibularina?. an ovoid test, a small number of coronal plates 

 surmounted by few and large primary tubercles, supporting 

 proportionally equally large primary radioles, simple rectili- 

 near poriferous zones, no petaloid ambulacra — in fact scarcely 

 one of the features we are accustomed to associate with the 



* The young of the following genera have served as a basis for the 

 preceding analysis of the embryonic stages of the Desmosticha : — Cidaris, 

 Dorocidaris, Goniocidaris, Arhacia, Porocidaris, Struw/ylocentrotus, Echi- 

 nometra, Echinus, Toxojmeustes, Hippono'e, Temnupleurus, Te?nnechmus, 

 and Trigonocidaris. 



