ECHINODERMATA. 287 



the small mamelons imperf orate; minute tubercles and granules 

 irregularly surround the primary tubercles, occuring in greatest 

 number along the median suture; poriferous zones slightly sinu- 

 ous, three or four pairs of pores in each plate arranged in a 

 curved series along the margin; pores unigeminal. Interambu- 

 lacral areas about one and one-half times as wide as the ambu- 

 lacral, with two rows of primary tubercles somewhat larger than 

 those of the ambulacra; the areolas very large, confluent above 

 and below ; throughout the greater part of the column the two 

 rows approach each other, but towards the apical disk they be- 

 come widely separated; the smaller tubercles are disposed in 

 more or less regular rows on either side of the primary series. 

 Oral opening small, occupying a little more than one-third of the 

 diameter of the test, irregularly pentagonal in outline. Discal 

 opening large, pentagonal in outline. 



The dimensions of a complete test are: width, 17.2 mm.; 

 height, 9.4 mm. 



Remarks. This species is very similar to Pseudodiadema 

 diatretum in general form, and without close observation the two 

 would not be separated. 'P. speciosum, however, is characterized 

 by its imperforate mamelons and by the larger areolas of the 

 interambulacral plates which are more completely confluent above 

 and below. Clark has placed the two species in different genera, 

 referring P. specwsuwh to the genus Coptosoma, but the differ- 

 ences between them seem to be no more than specific in import- 

 ance. The characters used to. distinguish the genera Pseudo- 

 diadema and Co'ptosoma are found in the arrangement of the 

 elements which constitute the compound ambulacral plates, Cop- 

 to soma having a larger number of demi -plates. In the two 

 species under discussion the number and arrangement of the 

 pores themselves in each ambulacral plate is essentially identical, 

 and the supposition would be that they are cogeneric, although 

 the actual sutures between the elements of these compound plates 

 have not been observed by the writer, nor have they been 

 described or figured by Clark. Co'ptosoma* ordinarily has a larger 

 number of pairs of pores in each ambulacral plate, consequently 

 these two species are both referred to Pseudodiadema. 



