On Cyrtoma, a new genus of Fossil Echinida. 171 



most perfectly radiated structure, every one of the numerous 

 pieces composing their complicated skeleton forming a 

 part either of the radius, or the surface of a spheroid, In 

 the Asterias, on the other hand, although we see the radiated 

 form more conspicuously developed in those branches on 

 which the starlike figure depends, yet when we come to 

 examine the structure of these animals more closely, we 

 perceive nothing of that beautiful mathematical order which 

 we observe in the Echinus. In the first place, the upper 

 and lower surfaces present two distinct kinds of structure, 

 in each of which the spheroidal disposition of the parts is 

 quite lost. In the next place, the plates composing the rays 

 are placed in rows which have no relation whatever to the 

 properties of a sphere, but a very considerable tendency to 

 the disposition of parts composing the annulosa, but whether 

 this be an analogy or an affinity, we are not sufficiently 

 prepared at present to decide. It is however important 

 to find amongst the Radiata a group which all natu- 

 ralists have agreed in placing at the head of the zoophytes, 

 and in which the radiated structure affords at least an ap- 

 proximation to the limbs of the higher classes, and still more 

 especially interesting when in this group alone, of all the 

 numerous tribes of polypes, and infusoria which follow 

 in the scale of nature, do we find any trace of a nervous 

 system. Next in importance to a nervous system having 

 been detected in the Asterias, is the fact of their having the 

 dorsal and abdominal regions distinctly marked. In the 

 Echinidas, as we have already remarked, M. Agassiz has the 

 merit of having pointed out for the first time a bilateral 

 disposition of the parts, but we have seen notwithstanding 

 how perfectly the spheroidal form is developed in every part 

 of their structure, and that below them in the scale no 

 tendency to the bilateral form has as yet been traced in the 

 animal kingdom. In ascending however to the higher forms, 

 we find not only the same signs of bilateral structure in 

 the Asterias, but the disc extended at certain points into 



2 a 



