POUKTALESIA TANWERI. 137 



circular shape of the actinal system of P. Tanneri is in great contrast to 

 its elongate elliptical outline in P. Jeffreysi. 1 Yet, in spite of this appar- 

 ent difference in outline, the number of coronal plates in the different 

 ambulacra or interambulacra does not vary perceptibly. 



The anal system of P. Tanneri is, on the contrary, much less flattened 

 than that of P. Jeffreysi; 2 Figs. 198, 199. The posterior edge of the lower 

 part of the anal system is strengthened by a low ridge running along the 

 inner part of the surrounding plates (PI. 57, fig. 4): The anal system is 

 divided into two parts, the lower somewhat pointed ; below the circular 

 anal opening it is covered by irregularly arranged plates larger than those 

 of the upper part, which is flattened, with rounded top and re-entering 

 sides, and is strengthened by more numerous smaller plates increasing in 

 size towards the anterior edge. 



An enlarged view of the abactinal system (PI. 56, figs. 5, 6) of two 

 specimens nearly of the same size, 16 and 17 mm. in length, Figs. 189, 190, 

 shows that the ocular plates have all disappeared, the ambulacra abutting 

 in the triviuin against the genital plates, and in the bivium against the 

 abactinal plates of the posterior interambulacra. There are four large 

 genital pores in the central disk, but only three genital plates could be 

 distinguished ; there are four in P. hguncula. 



The bivium and trivium are separated by the abactinal plates of the 

 posterior interambulacra, and in addition by two plates placed one in front 

 of the other in the extension of the abactinal plates of the odd posterior 

 interambulacrum (PI. 56, fig. 5). In another somewhat younger specimen 

 (PI. 56, fig. <j) the arrangement and number of the intercalated plates is 

 very different ; at the abactinal extremity of the paired plates of the odd 

 interambulacrum there are no less than three small plates, the largest of 

 which connects with two plates having much the same relation to the 

 posterior interambulacra] plates which they held in the other specimen 

 (PL 56, fig. £), so that we might really consider all these plates as the 

 irregular extension of the abactinal plates of the odd interambulacrum. 

 Yet this may not be the case, as in P. Jeffreysi the three additional plates 

 are not joined with the odd interambulacrum, as in P. Tanneri (PI. 56, fig. 6'), 

 but are placed next to the genitals, 3 and the two intermediate plates are 

 isolated from the single plate adjoining the extremity, Fig. 200, of the 



1 Loven, Pourtalesia, PI. IV, figs. 18,20. 2 Id., PI. I, fig. 4. 



3 Id., Pis. I, fig. 1; V, figs. 25-28. 



