HEMIPEDINA CUBENSIS. 291 



TRIPLECHINIDAE. 



Subfamily Triplechinidae A. Agass. 



(PSEUD0D1ADEMA.) Hemipedina. 



Hemipedina Wright, 1855. Brit. Ool. Eclrin. 



The small number of living specimens of the genera belonging to 

 Pseudodiadematidae thus far found renders it extremely difficult to form 

 any opinion of the characters hitherto employed to distinguish the genera 

 allied to Phymosoma and Pseudodiadema. It seems pretty evident that there 

 have been too many generic divisions introduced, but I am unable from the 

 material thus far collected to give any satisfactory limitation of the genera of 

 Pseudodiadematidae, resting as they do upon the presence or absence of crenu- 

 lation and perforation of the tubercles. For this reason I prefer, until better 

 characters can be pointed out than those upon which I based my separations 

 from Hemipedina of Caenopedina, to regard this genus as identical with 

 Hemipedina, regarding Hemipedina at the same time as a subgenus of 

 Pseudodiadema, from which it differs only in having smooth tubercles, not 

 crenulate, and having deep actinal cuts and smaller ambulacral tubercles. 



The genus contains Echini having but two principal vertical rows of tuber- 

 cles in the ambulacral and interambulacral areas, the ambulacral tubercles 

 smallest, both perforate but not crenulate. Poriferous zones simple, forming 

 arching curves round the primary tubercles. Actinal cuts marked, ten large 

 buccal plates. Abactinal system large, not prominent. Spines long, slender, 

 longitudinally striated. The poriferous zone resembles that of Orthopsis, 

 having also, like it and Echinopsis, perforate but not crenulate tubercles. It 

 resembles Acrosalenia as Avell as Phymosoma, of which the tubercles are, 

 however, crenulate and not perforate. 



Hemipedina cubensis 



! Caenopedina cubensis A. Agass. 1869. Bull. M. C. Z., I. 

 ! Hemipedina cubensis A. Agass. 1872. Rev. Ech. Pt. I. 



PI. III. f. 1-7. 



This species is a living representative of the genus Hemipedina of Wright 

 (as emended by Desor, Wright having included in it species of other genera 

 of Pseudodiadematidae). It differs from its fossil representative by the pecu- 

 liar arrangement of the pores, which have a tendency to arrange themselves 



