444 A. Olsson — New Genus of Palceechinoidea. 



size and shape of the adambulacrals cannot be ascertained 

 because of the overlapping of the second column of plates ; 

 they appear, however, to be rhombic or pentagonal in shape 

 and slightly smaller than the second column of plates, which 

 are undoubtedly the largest of the interambulacrals in this 

 species. The remainder of the interambulacral plates are 

 mainly hexagonal in shape, quite perfectly so near the dorsal 

 surface, but on passing ventrally their edges become rounded. 

 On the extreme oral surface the adambulacral plates are small 

 and scale-like, closely resembling those of Lepidechinus. 

 Along a line through the initial plate 4', the diameters through 

 these plates are as follows : 2*75 mm , 4 mm , 3 mra , 3 mm , 3'75 mm , and 

 2'75 mm . The number of columns of interambulacrals is vari- 

 able even in the same genus and may, therefore, only be con- 

 sidered of specific value. The list below gives the range of 

 the number of columns of interambulacrals in the various 

 species in the genera of the family Lepidocentridae. 



(Eu) Lepidocentrus rhenanus Schultze 5 I 

 (Eu) " midleri Beyr 111 



(Eu) " eifiliamis Miiller unknown 



(Eu) Perischodomus biserialis McCoy 5 I 



" illinoisensis Worthern & Miller 



5 or more I 

 Lepidechinus rarispinus Hall 111 

 " imbricatus Hall 8 I 



Lepidechinoides ithacensis 6 I 



In the number of ambulacral plates to each of the adambu- 

 lacrals there is some variation. As could be expected both in 

 Lepidocentrus and Perischodomus, in which the adambu- 

 lacrals are large — being the largest of the interambulacrals — 

 there are many ambulacrals to each of the adambulacrals, 

 while in Lepidechinus and in Lepidechinoides, in which the 

 adambulacrals are relatively small, the opposite is the case, as 

 shown below. 



Lepidocentrus rhenanus Schultze 7-8 A 



" mulleri Beyr 7-8 A 



" eifllianus Miiller unknown 



Perischodomus biserialis McCoy 5-8 or more A 



<• illinoisensis Worthen & Miller 



Lepidechinus rarispinus Hall 3-4 A 



" imbricatus Hall 



Lepidechinoides ithacensis 3-4 A 



In the above brief summation of a few of the generic char- 

 acters, it is evident that it is fairly close to Lepidechinus Hall. 

 It has for one of its most important differences the perforation 

 of the ambulacral plates in the center and not on the distal 



