354 Prof. A. Agassiz on Palceontological 



at any geological horizon. On taking in succession the modi- 

 fications undergone by the different parts of the test, we can 

 trace each one singly, without the endless complication of 

 combinations which any attempt to trace the whole of any 

 special generic combination would imply. 



Leaving out of the question for the moment the Palsechi- 

 nidse, Ave find no difficulty in tracing the history of the cha- 

 racters of the genera of the regular Echini which have 

 existed from the time of the Trias and are now living, pro- 

 vided we take up each character independently. Nothing 

 can be more direct than the gradual modification of the 

 simple, barely undulating poriferous zone, made up of nume- 

 rous ambulacral plates covered by granules, such as we find 

 it among the Cidaridse of the Trias, first into the slightly 

 undulating poriferous zone of the Hemicidaridse, next into the 

 indistinct arcs of pores of the Pseudodiadematidse, then into 

 the arcs with a limited number of pores of the Triplechinidse, 

 and finally to the polyporous arcs of the Echinometrada3. 

 What can be more direct than the gradual modification to be 

 traced in the development of the primary ambulacral tuber- 

 cles, such as are characteristic of the Echinidse of the present 

 day, from their first appearance at the oral extremity of the 

 ambulacral system of the Hemicidaridse, and the increase in 

 the number of primary interambulacral tubercles, accom- 

 panied by the growth of secondaries and miliaries, which we 

 can trace in Hemicidaris^ Acrosalenia, and Stomechiniis, 

 the increase in number of primary and secondary tubercles 

 being accompanied by a reduction in the size of the radioles 

 and a greater uniformity in their size and shape ? 



But while these modifications take place, the original 

 structural feature may be retained in an allied group. Thus 

 the Cidaridse retain unchanged from the earliest time to the 

 present day the few primary tubercles, the secondary granules, 

 the simple poriferous zone, the imbricating actinal system, 

 and the few coronal plates, with the large apical system and 

 many-shaped radioles ; while in the Salenidse the primary 

 interambulacral tubercles, the secondary granules, the radioles, 

 the genital ring are recognized features of the Cidaridse, 

 associated, however, with an Echinid actinal and anal system, 

 Hemicidarid primary ambulacral tubercles, and an Echinid 

 poriferous zone. In the same way, in the Diadematidse, the 

 large primary interambulacral tubercles are Cidaridian fea- 

 tures, while the structure of the ambulacral tubercles is 

 Hemicidaridian. The existence of two kinds of spines is 

 another Cidaridian feature, while the apical and actinal 

 systems have become modified in the same direction as that 



