444 PE.OP. p. M. DUNCAN 02f THE STETJCTITRE OP 



(Echinides du Depart, de la Sarthe, par Cotteau et Triger, 1859, 

 plate xxxiv. fig. 5), the propriety of calling the species a I)i_plopodia 

 is doubtful ; for the pairs of pores resemble in their arrangement 

 those of Pedina. 



The genus CypTiosoma, Agass., is characterized by the doubling of 

 the pairs of pores near the apex ; and this character is certainly 

 developed with varying age according to the species. But the adults 

 have the character, and tee forms which are without it are placed in 

 the genus Coptosoma, Desor. The alliance of CypTiosoma to the 

 Pseudodiadema-gTou]) is evident from the common external characters, 

 and there are points in the structure of the ambulacra which unite 

 the genera and at the same time refer to the Triplechinidse. 



DiPLOPODiA VEESiPOEA, Phill., sp., uou Diplopodia subangularis, 

 McCoy. 



This species, usually attributed to Pseudodiadema, is ver}- familiar 

 to the students of the fossils of the Coralline Oolite, and it has been 

 figured by Dr. Wright (op. cit. plate vii. fig. 4). 



The nature of the construction of the ambulacra has not been 

 given, except in a general manner. 



The diplopodous condition of the pairs of pores is well worthy 

 of study, and it can be examined in a specimen in mj possession. 



Taking one of the zones of an ambulacrum, it is seen that the 

 first plate next to and in contact with the radial plate is a low and 

 narrow primary, with the adoral pore close to the median line of 

 the ambulacrum, and the aboral placed obliquely to it, the first- 

 mentioned pore being also on the line of suture between the first and 

 second plates (fig. 18). The second plate is at least twice the size of 



Fig. 18 (see p. 452). 



s2 



the other, is slightly higher and much broader, and the adoral 

 pore reaches the suture between this and the third plate, being 

 placed almost directly actinally to the aboral pore of the first 

 plate. The aboral pore is far out of the vertical line of the corre- 

 sponding pore of the first plate. Both of the plates are primaries, 

 and do not form a compound plate. The third plate is high at the 

 median line and very low at the ambulacro-interradial suture ; the 



