226 R. T. JACKSON STUDIES OP PALJSECHINOIDEA. 



207). Tlie other species, P. biserialis, M'Coy, has been well illustrated 

 by Keepino; (2'2). The type of M'Coy's and Keeping's researches on 

 this species is in the Woodwardian Museum at Cambridge, England. 

 A specimen of this second species from the Carboniferous of Hook Point, 

 county Wexford, Ireland, is in the Museum of Comparative Zoology 

 (catalogue number 3071). The specimen consists of a portion of an in- 

 terambulacrum and a single ambulacral plate. The plates are rounded 

 on their borders and imbricate strongl}'- aborally, as described. The in- 

 troduction of one column of interambulacral plates, the fifth, is shown. 



STRUCTURE AND DEVELOPMENT OF LEPIDECHINUS. 



The genus Lepidechinus was founded by Professor Hall (16) for the 

 rece]:>tion of a species with imbricating i)lates. Lepidechinus imbricatus, 

 Hall, the type of the genus, has not been figured or 2:>ublished from orig- 

 inal observations, so far as I can find out, except in the original descrip- 

 tion. Another species, Lepidechinus rarispinus, was later described by 

 Hall (17) and a figure given. All the known specimens are sandstone 

 casts. Jaws have not been previously described in the genus. 



Professor C. E. Beecher, of New Haven, kindly loaned me a valuable 

 specimen of Lepidechinus rar^ispinus from the Waverly sandstone. Sub- 

 carboniferous, of Warren, Pennsylvania. This choice specimen (plate 7, 

 figure 42) is a cast of the ventral side. One interambulacrum is very 

 clear, and it with adjacent ambulacra, jaws, and some plates of the peri- 

 stome are shown in the accompanying figure. 



At the ventral border of the interambulacrum there is a single initial 

 plate, V (plate 7, figure 42). This plate has a sharp apex dorsally, two 

 inclined, but very short sides at the contact with the bordering ambu- 

 lacra, and ventrally is truncate. Lepidechinus and Pholidocidaris (plate 9, 

 figure 54) are the only genera of Paleozoic echinoids in which I have seen 

 the single initial plate, although every evidence of its presence has been 

 observed in Melonites (plate 3, figure 10) and 0/i<7opor?/.s (plate 6, figure 26). 

 In Melonites and Oligoporus the supposed ventral plate with part of the 

 ventral border of the two succeeding plates was removed by resorption ; 

 but here we have a case in which resorption at most removed only part 

 of the initial plate of the area. In Lepidechinus the second row of inter- 

 ambulacral plates consists of two quite irregular plates, 1 and 2. Part 

 of this irregularity may be due to imperfections of preservation, as the 

 outlines, especially of plate 2, are quite difficult to make out in this sand- 

 stone cast. Plates 1 and 2, by addition of succeeding plates, form the two 

 adambulacral columns, as in Melonites (plate 2, figure 2). These adambu- 

 lacral plates, on account of the peculiar scale-like form of the plates, are 



