114 



PALEOZOIC ECHINODERMATA. 



Agassiz and Desor in the ' Annales des Sc. Nat/ for November 

 1846, no mention is made of the genus Echinocrinus, but the 

 species which were to have formed the type of it (Cidaris Nerii, 

 &c.) are given under the new title of Palceocidaris, which of 

 course has no claims for adoption on the score of priority; nor 

 do MM. Agassiz and Desor even there seem aware of the pecu- 

 liarity in form of the interambulacral plates or their abnormal 

 number, although my observations on those points are mentioned 

 by M. Verneuil nearly two years before in his ' Coup d^oeil general 

 sur la Faune Paleozoique de Russie/ prefixed to the second vol. 

 of MM. Murchison, Verneuil, and Keyserling^s great work on 

 Russia and the Ural Mountains. Under those circumstances, 

 therefore, it seems the most simple and correct course to use the 

 term Archceocidaris for those fossils. 



The order Perischoechinida may be divided into two families : 

 1st, PalcBchinidce, having the interambulacral plates crowded with 

 small, subequal, spinigerous tubercles, not perforated, the spines 

 of one form (including Palaechinus, Melonites, Owen and Nor- 

 wood, &c.); 2nd, Arch(2ocidarid(B, having the spines and tubercles 

 of two forms and sizes, the primary spines very large, generally 

 muricated, crenulated at the base, and each supported on a large 

 mammillated and perforated primary tubercle surrounded by an 

 elevated ring, never more than one on any plate, generally sur- 

 rounded by a crowd of the small secondary tubercles (including 

 Archceocidaris J M^Coy, and the following). These family divi- 

 sions rest on the same characters as the separation of the true 

 Echini and the Cidaridce among the normally formed Echinida, 



Perischodomus (M^Coy), n. g. 



Etym. Trepio-'x^cov, complexus, and Bm/jlu, domus. 



Gen. Char. Spheroidal, depressed, subpentagonal ; ambulacra 



a. Diagram of portions of interambulacrum and ambulacra of Perischodomus. 



b. One of the primary and some of the secondary tubercles magnified more 



highly. 



c. One of the ovarian plates. 



