512 PALAEONTOLOGY OF ILLINOIS. 



PHOLIDOCIDAKIS IRREGULARIS, M. and W. 



Pholidocidaris irregularis, MEEK and WOUTHEX. Proceed. Acad. Nat. Sci., Phila., 1869, p. 78. 



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 ambulacra at that 

 point. These six ranges, as they lie flattened by pressure, 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 cor- 

 rect idea of their outlines, 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 second- 

 ary 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 adjoining them, and in some instances measure 

 1 inch in length, and about 0.60 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 iuterambulacral series. In one crushed speci- 

 men, showing a part of the fossil composing apparently the under side, 

 extending an inch or more away from the supposed oral opening, por- 

 tions of three of the ambulacral and two of the interambulacral series 

 of plates are seen, apparently nearly in their relative positions, except- 

 ing that they are all spread almost on a plane, and more or less dis- 



