FOSSILS OF THE KKOIU'K (iKOTT. 5H 



pression ur prominence around the base; 011 the upper (?) side of the 

 lo<ly only the marginal rows provided with primary tubercles. Am- 

 bulacral areas \\ ide. and occupied by six or more irregular rows of un- 

 equal, irregular pieces, some of which are as large as the smaller inter- 

 ambnlacral plates, and all strongly imbricating in the opposite direction 

 from the interambulacral scries : each pierced by two pores, and the 

 larger ones usually marked with one or two additional pits, which, with 

 the pores, are surrounded by a large circular impression, while some of 

 them sometimes show a tubercle intermediate in size between the pri- 

 mary and secondary series. Apical disc unknown, but a single rather 

 large plate, believed to belong to it. is seen to have six or seven pores 

 circling around near its outer margin, and a small tubercle in the mid- 

 dle. 



This type is related to several of the other palaeozoic genera, but can- 

 uot be properly referred to any of them. In the great irregularity of 

 its interambulacral plates, both in size and form, as well as in the ab- 

 sence of primary spines, excepting on the marginal rows, (at least on 

 the upper (?) side of the body), it seems to be very similar to PeriscJto- 

 (JimiXfi. of McCoy. It differs, however, clearly from, that group in the 

 much greater breadth of its ambulacra! areas, the greater number of 

 ranges of ambulacra! pieces, and their greater irregularity, as well as 

 in the much larger sizes of some of them, and their peculiar circular 

 impression around the two pores. If Perischodomns has its plates not 

 imbricating, that would also be another important difference : but al- 

 though that character is not mentioned in the description, we suspect 

 it may really exist, because \ve find it to occur in all the analogous types 

 in this country. 



Fr.'iu Lepideclrinm it is also readily distinguished by the much greater 

 breadth of its ambulacral areas, and its more numerous rows of ambula- 

 cra! pieces and pores, as well as by the larger size and the other pecu- 

 liarities of these species. 



In the breadth of its ambulacral areas, and the number of rows of 

 pieces occupying the same, it is more nearly related to our Lepidesthes, 

 but it differs in the great irregularity of these pieces, both in size and 

 form, as well as in the curious circular impressions of the same; also in 

 the possession of large primary tubercles and spines on some of the in- 

 terambulacral pieces. 



It is probably more nearly related to the form referred in this paper, 

 doubtfully, to E<.n:'l,irix. but it differs materially in the much greater 

 breadth of its ambulacral areas, more numerous ranges of ambulacral 

 plates, and the larger sizes, and other peculiarities of these species, as 

 well as in not having primary spines and tubercles on all of its iuter- 

 ambulacral plates. 



