1 70 On Cyrtoma, a new genus of Fossil Echinida. 



between analogy and affinity, we can expect nothing but 

 additional confusion from the mere alteration of names and 

 groups.* 



With regard to the situation of the Echinida in the animal 

 kingdom, M. Cuvier placed them next the starfish, at the 

 head of his fourth or last division Radiata, which embraces 

 at least two very distinct classes of animals, one of which, 

 the Acrita of MacLeay, presents but little of the radiated 

 structure. Even the term Echinodermata which was ap- 

 plied by Klein to the Echini of Linnaeus, and intended only 

 for radiated animals whose bodies are incrusted and covered 

 by prickles, has been extended by Cuvier to a whole class 

 of animals, including the intestinal worms, the polypes, 

 sponges, and infusoria. M. de Blainville has restricted the 

 name to the Holothuriae, Echinidae, and Asteridae, which 

 nevertheless are three very extensive groups. With re- 

 gard to the first I have no personal knowledge, being merely 

 acquainted with them, from the description of authors. Ac- 

 cording to M. de Blainville, M. Bianchi was the first to detect 

 the affinity of Holothuriae to Echini, and he even names one of 

 them Echinus coriaceus, but Pallas and some others consider 

 that they ought to be placed in the class of Polypes near 

 Actinia. Asteriae appear to be more perfectly developed 

 animals than those of either of the other two groups, and 

 therefore deserve a foremost place, as well on account of 

 a nervous system having been discovered in some of the 

 species, as from the fact of the pieces composing their 

 testa approximating to the structure of articulated ani- 

 mals. In the Echini, which of all Radiata present the 



* The difference between the new genus Diadema, Gray, and the old genus 

 Cidaris, is chiefly that the spines are smaller and more numerous ; again, the 

 genus Astropyga differs from Cidaris in the oviducal plates being longer, and 

 the genus Arabacia is made to differ from Diadema in having no perforation 

 in the large manimilae. Now we require to be informed why such slight peculi- 

 arities should be considered of sufficient importance for the construction of 

 genera as Ophiocoma, Phytocrinus, Ganymeda, Melocrinus. We luckily get 

 rid of the genus Diadema, from the term having been previously applied to a 

 genus of Cirvhopoda by Ranzani. 



