ECHINOIDEA. 15 



or 8a. On the other hand, the figures in the Pal^ontologie frangaise, for instance 

 plate 1201, fig. 4i, or plate 1203, fig. 6 — show distinctly that the triple groups of pairs 

 of pores are formed not by the pores of one ambulacral plate, but by the pores of 

 two consecutive plates. If we call the pairs of pores on one plate by numbers 1, 2, 3, 

 1 being the apical, 2 the median, 3 the peristomal pair, we find that the groups are 

 composed of numbers 3, 1, 2, while the pairs on each plate are arranged in such a 

 way that they form an irregular triangle, pair No. 2 being the most external, 

 No. 3 the most internal one, and taken as a whole the pores are arranged in three 

 rows, the external of which is formed by pairs No. 2, the median by pairs No. 1, 

 and the internal one by pairs No. 3. 



Now, in the specimen described below it will be noticed that the pores are 

 seemingly arranged in groups of two, which are inclined ia the direction of the 

 peristome ; the third pair, however, is so much reduced that it is hardly visible, 

 but if of the same size as the former ones it would be the lowest in a group, which 

 consists of pairs 2, 3, 1, of which 2 and 3 belong to one, 1 to the next following 

 plate in the direction of the peristome. On each plate the pores are therefore 

 arranged in such a way that pair 1 is the most internal pair, 2 the most external, 

 and pair 3 the median one. The following diagram will show the arrangement of 

 the pores on each plate, the arrangement of the pairs in triple groups, and the 

 direction of inclination of each group in the genus Psammechinus and Frotechinus. 



The difference in the arrangement of the pairs of pores in the two genera is too 

 conspicuous to be overlooked, and I think that it fully justifies the establishment 

 of a new genus, considering besides that pair No. 1 has become obsolete. 



How far the above rule as to the position of the pairs of pores on each ambu- 

 lacral plate and their arrangement into groups applies to the other genera of the 

 OUgopori, I am unable say for the present, but to judge from Stomechinua denuda- 

 tus, Cott. (Pal. franc, pi. 1204, fig. 4), this genus differs from the above two genera 

 by a different arrangement ofthe pores, which are arranged in pairs inclined towards 

 the peristome, ia the order of 3, 1, 2, so that pairs No. 2 form the internal, 3 the 

 external, and 1 the median row. 



