174 On Cyrtoma, a new genus of Fossil Echinida. 



disc, and each surface is marked by a rosette peculiar to it- 

 self, the lower rosette still evincing an affinity to the fissures 

 in the lower surface of Asterias. This latter character is 

 strongly marked in our new genus Cyrtoma, which appears 

 to follow the Scutellse, and by its more elevated summit to 

 connect that genus with Echinus, from which we pass to 

 other forms without a distinct rosette on the base, as the 

 Nucleolites, in which the disc itself becomes raised, still 

 however presenting the elevated figure. The Nucleolites 

 lead directly to the Galerites, and these last to the Spa- 

 tangi. In the Echini we have the most perfect spheroidal 

 forms, as well as the most perfect radiated structure, as 

 far as the skeleton is concerned, of all the other Echino- 

 dermata. Here we- find the peculiar ambulacral grooves, 

 which were confined alone to the dorsal surface of the 

 star-fish, converted into double bands of tubes, passing like 

 the meridians of a globe from the base to the apex. In 

 the absence of limbs extended from the disc, as in the 

 star-fish, we find every part of the round and apparent- 

 ly helpless body covered with jointed spines which can 

 be turned in any direction from which danger threatens, 

 or wants may be supplied. The only difference between 

 these animals and the Cidarites is, that in the latter the 

 articulating pustules or tubercles for the joints of the spines 

 have each a depression in the centre, probably for a 

 ligament. Their separation on this account from the 

 Echini is at least highly useful, the species of both groups 

 being so very numerous as not to be distinguished without 

 difficulty, but for this convenient subdivision. We observe 

 at the extremities of the Echini and Cidarites (which are 

 both regarded as one natural group) remarkable variations 

 both in the axis and diameter of the body, which have 

 led to the formation of many new genera, as Diadema* 

 and Arbacia, when the shell is depressed, Echinometra 



* This term, though adopted by De Blainville, Gray, and Agassiz, has been 

 nevertheless appropriated before for a genus of Cirrhopoda, as already observed 

 page 169. 



