172 SYNOPSIS OF THE CHARACTERS OF THE 



specimens, and tlie discovery of two more new species from the shales of the north and south of Ireland, have 

 enabled me to give the above generic characters. I have adopted the name proposed by Dr. Scouler. Most of 

 the Echinidee have the interambulacral spaces composed but of two rows of large, pentagonal plates ; in the pre- 

 sent o-enus, on the contrary, it is composed of from five to seven rows of plates ; the spiniferous tubercles and 

 ambulacra have the same character as in Echinus. 



Pal^echinus elegans. M'Coy. (PI. XXIV. fig. 2). 



Sp. Ch. — Ovate ; dorsal end pointed ; ventral end obtuse, rounded; ambulacra large, convex; interam- 

 bulacra composed of two rows of pentagonal, and three rows of hexagonal plates ; interambulacral plates mam- 

 millated, and covered with very minute spiniferous tubercles, width of the lateral interambulacral plates equal 

 to five ambulacral ones. 



This species is readily distinguished from the P. ellipticus, by its pointed, superior extremity, the mam- 

 millated surface of the plates, independent of the spiniferous tubercles, and the greater proportional size of the 

 ambulacral plates. Length about two inches. 



Pal^chinus ellipticus. Sc. MSS. (PL XXIV. fig. 3). 



Sp. Ch. — Elliptical; ventral and dorsal ends equal; ambulacra prominent, convex, and having two rows 

 of pores on each side; interambulacra composed of two rows of pentagonal, and three rows of hexagonal plates ; 

 plates evenly convex, regularly marked with numerous minute tubercles, nearly in quincunx ; the width of 

 one pentagonal, lateral, interambulacral plate equal to seven ambulacral ones. 



This species is distinguished by its regular elliptical form, and the small size of its ambulacral plates, seven 

 of which are only equal in width to one of the interambulacral ones. Length two inches, width one inch eight 

 lines. 



Pal^chinus gigas. M'Col/. (PI. XXIV. fig. 4). 



Sp. Ch. — Orbicular, depressed; interambulacra composed of six rows of plates; four rows of pores on each 

 side the ambulacra ; seven ambulacral plates equal to the width of one of the lateral anambulacral ones ; tubercles 

 large, equal, each surrounded by a ring. 



This is by far the largest species of the genus 1 have met with. The above specific characters will distin- 

 guish it readily from its congeners ; the spiniferous tubercles on the plates are nearly equal in size, and disposed 

 regularly in quincunx ; each tubercle is of considerable size, convex, imperforate, and surrounded by a narrow, 

 elevated ring ; the space between the tubercles is smooth ; the ambulacral plates bear tubercles in this species, 

 as well as the interambulacral, a character which may be useful in distinguishing fragments from the other spe- 

 cies. Length of a hexagonal plate from the middle of the interambulacra four lines, width six lines. 



PALiEGHINUS (?) KONIGII. ilf'Coj/. (PI. XXIV. fig. 10). 



Sp. Ch. — Plates, with numerous, irregularly placed, large, compressed tubercles, between which are nume- 

 rous, small, simple tubercles, very irregularly disposed. 



This species is more nearly allied to Echinus, in the character of its tubercixlation, than any other of the 

 Palcechini ; in the latter genus the tubercles are uniform, or nearly so, in size and structure, while the present 

 species resembles Echinus, in having two distinct sets of tubercles, the smaller being greatly more numerous. 

 The surface in the specimens I have examined was reticulated with very fine lines, apparently the traces of 

 muscular action of the integument for moving the spines. 



PAL^CHmus sPHiEBicus. Scouler. MSS. (PI. XXIV. fig. 5). 



Sp. Ch. — Spherical; anal and oral ends equal; ambulacra prominent, convex, with two rows of pores on 

 each side ; interambulacra composed of two rows of pentagonal, and in some cases four, in others five rows of 



