448 PEOr. p. MAUTIN DUNCAN ON THE 



indistinctly crenulated. The sutural impressions in the shape 

 of deep disconnected pits, occurring not only in the angles of 

 the plates, but sometimes three or four, even six, in a horizontal 

 suture." 



The species Pleurechinus hothryoides, L. Agassiz, was founded 

 on a solitary specimen, which has a denuded test. A. Agassiz 

 states, in the ' Eevision,' p. 46'5 : — " It is unfortunately in such a 

 condition that no specific description of any value can be made, 

 and 1 can do nothing except to call attention to the species, 

 totally unlike, as far as it goes, any other species of Temno- 

 pleuridae known to me." There are four deep disconnected pits 

 of about equal size along the sutures of the plates above the 

 ambitus, and there are two pits in the ambulacral area. The 

 poriferous zone is narrow and undulating. In the interradial 

 areas, the primary tubercles form three vertical rows and are of 

 uniform size ; there are two outer rows of smaller tubercles 

 separating the pits from the poriferous zone. The pits in the 

 interradial areas are separated by the primary tubercles. In the 

 ambulacra there are two principal outer vertical rows of primary 

 tubercles, and two irregular median vertical rows of smaller 

 tubercles. The test of the species is quite high, ovoid, with an 

 outline recalling somewhat Aniblypneustes. 



Other forms were confounded with this species ; but it is 

 quite clear that no more than four pits occur in horizontal series 

 in the interradials, and two in the ambulacra. The condition of 

 the specimen probably prevented a distinct statement regarding 

 the crenulation of the tubercles. The specimen was wrongly 

 stated to have been derived from the Galapagos Islands ; but 

 Alex. Agassiz very properly remarks that it must have come from 

 the East-Indian archipelago or the Philippines. 



Alex. Agassiz states * that " this genus corresponds to the 

 genus OpecJiinus, Desor, which was established to receive several 

 very characteristic fossil species of Temnopleuridse, which 

 D'Archiac and Haime distributed ia TemnecMnus and in Tem- 

 nopleiirus" In order to clear the way for placing the genus 

 PleurecJiinus in its proper position, and to give it a proper clas- 

 sificatory value, it is necessary to consider this statement first 

 of all. 



OpecJiinus, Desor (1858), includes the above-mentioned Echini 

 described by D'Archiac and Haime, and a small specimen forming 

 * ' Revision of the Echiui,' p. 465. 



