Mr. F. M'Coy on some new Palaozoic Echinodermata. 253 



Russia and the Ural Mountains. Under those circumstances, 

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

 term Archaocidaris for those fossils. 



The order Perischoechinida may be divided into two families : 

 1st, Palofchinida, 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.) j 2nd, Archceocidaridoi, 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, M^Coy, and the following). These family divi- 

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

 Echini and the Cidarida among the normally formed Echinida. 



Perischodomus (M'Coy), n. g. 



Etym. irepLa^oov, complexus, and Bco/Jia, domus. 



Gen. Char. Spheroidal, depressed, subpentagonal ; ambulacra 

 narrow, of two rows of small plates, most usually of a trans- 



m 



^ 



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



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



higlily. 



c. One of the ovarian plates. 



versely elongate pentagonal figure, and each pierced by one 

 pair of simple pores; interambulacra wide, of five rows of 

 plates very irregular in size and shape, all the plates covered 

 with small equal granules or secondary tubercles, while the 

 row on each side adjoining the ambulacra alone bear the small, 

 smooth primary spines, one on each, the supporting tubercle 

 being small, mammillated, perforated, but not crenulated, sur- 

 rounded by a double ring and situated not in the centre, but 

 near the ambulacral edge, a little above the middle ; ovarian 



