78 PROCEEDINGS OF THE ACABEMY OP 



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 interambulacral pieces. 



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

 fullj' to Eocidaris, 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 pecularities of these species ; as well as in not having primary 

 spines and tubercles on all of its interambulacral plates. 



Note. — After preparing the foregoing description, with the view of propos- 

 ing a new genus for the reception of this fossil, our attention was called by 

 Prof. Alexander Agassiz to a very similar type, that was published by Miiller 

 under the name Lepidoccntrus^ in 1856, from the Eifel Limestone (Aus den 

 Abhandlungen der Konigl. Akad. der Wissenschaften zu Berlin, p. 258, taf. iii) ; 

 but which has been entirely overlooked by subsequent authors. After com- 

 paring our specimens with the figures and description in Miiller's paper (a 

 copy of which Prof. Agassiz was kind enough to loan us), we find our type 

 agrees so nearly with his genus, in all the parts known to him, that we have 

 concluded to refer it provisionally to the same. Miiller, however, knew no- 

 thing of the nature of the ambulacral pieces of his type, and we have there- 

 fore no means of comparing these important parts. As it is already known 

 that there are various genera of these older types, agreeing in some characters 

 and differing in others, it is quite probable our fossil may belong to a distinct 

 genus. If so, we would propose to call it Pholidocidaris. 



Lepidocentrus iRREGuiiARis, M. and W. 



The specimens of this fossil that we have had an opportunity to study are 

 too much crushed and broken to give a clear idea of its general form, or to 

 admit of being systematically described. It seems to have attained a rather 

 large size, however, and if of a depressed subglobose form, may even have 

 measured as much as three and a half to four inches in its transverse diameter. 

 Some specimens show from five to six ranges of interambulacral plates lying 

 together, so as to indicate that there were at least that many ranges between 

 the ambulacra at that point. These six ranges, as they lie flattened by pres- 

 sure, measure about two inches across. 



All of these interambulacral plates are thin and sharp at the edges, and of 

 only moderate thickness in the central region, while they present such a variety 

 of forms that it would scarcely be possible to give a correct idea of their out- 

 lines, without describing each individual plate. They are generally a little 

 longer than wide, however, and on what appears to be a part of the body below 

 the middle, most of them have the primary tubercles more or less distinctly 

 developed, though on some they are obsolete, or not easily distinguished from 

 the obscure secondary ones. 



On what appears to be the upper side of the body, no traces of any but the 

 small secondary tubercles covering the surface of all of these plates are visible, 

 excepting, as already stated, on those of the marginal rows. These marginal 

 plates on this side, are generally each as large as three or four of those adjoin- 

 ing them, and in some instances measure 1 inch in length, and about 060 

 inch in breadth, being of an elliptic form. The primary tubercle of each is 

 placed about midway of the length, and between the middle and the ambulacral 

 side. In many instances these tubercles are rounded off, as if the spines had 

 been dropped during the life of the animal, and the tubercles partly absorbed 

 away. 



The ambulacral plates are apparently even more irregular in size and form 

 than those of the interambulacral series. In one crushed specimen, showing 

 a part of the fossil composing apparently the under side, extending an inch 

 or more away from the supposed oral opening, portions of three of the ambu- 

 lacral and two of the interambulacral series of plates are seen, apparently 



[April, 



