Mr. F. M'Coy on some new Paleozoic Echinodermata. 251 



form a truncated disc) instead of being continued through a 

 sHt in the supra-basal plates nearly to their base. On the nature 

 of the peculiar sulcation, represented in the subjoined sketch in 

 four of the interambulacral spaces, I have no remark to offer. In 

 Prof. Forbes^ s paper on the British Cystidea in the second volume 

 of the ' Memoirs of the Geol. Survey/ p. 529, there is a figure 

 representing " the projection of the arm-bearing surface of the 

 Pentremites pentagonalis" which resembles the disc of our genus 

 except in having the posterior interambulacral space sulcated, and 

 with a thick mesial ridge like the rest ; I do not suppose that that 

 figure is meant to represent the Platycrinus pentagonalis of Mil- 

 ler, forming the Pentremites id. of G. Sowerby and Phillips, which 

 presents no resemblance of the kind. I only know the following 

 two species, from the carboniferous limestone. 



Cadaster acutus (M'Coy). 

 Sp. Char. Pelvic and supra-basal plates of equal length ; pelvis 

 acutely conical, obtusely subtrigonal in section ; columnar ad- 

 herence small, round, prominent; surface smooth. Length 

 6 lines, width of disc 5 lines. 



Not very uncommon in the carboniferous limestone of BoUand. 

 (Col. University of Cambridge.) 



Codaster trilobatus (M'Coy). 

 Sp. Char. Supra-basal one-third longer than the basal or pelvic 

 plates ; pelvis divided into three tumid lobes which hang be- 

 low the columnar adherence ; surface smooth. Length 7 lines, 

 width of disc 5 lines. 



Not uncommon in the carboniferous limestone of Derbyshire. 

 [Col. University of Cambridge.) 



Ord. Perischoechinida (M'Coy). 



All the known Echinida — from the spheroidal Echini with the 

 mouth and anus both central, one vertically under the other, to the 

 elongated, symmetrical Spatangi with their mouth and anus at 

 opposite ends of the ventral disc — all agree in having their case 

 made up of twenty vertical rows of plates, ten ambulacral and ten 

 interambulacral. This is not only the most persistent character 

 of the entire group, but the number becomes of extreme interest 

 when, with Agassiz and Valentin, we view the globose test of the 

 sea-urchins as a mere modification of the same parts which we find 

 in a 5 -rayed starfish, — an ideal division of the mesial suture con- 

 necting the two rows of plates in each interambulacrum of the 

 former, giving at once the ambulacra, lateral ossicles, and other 

 characters of the latter. The Echinites of the palseozoic rocks 

 however are constructed-on an entirely different plan, having three 



