ECHINOIDEA. 19 



become so small that they are hardly visible. On the ambulacral areas there are 

 six rows of them which are reduced to two near the mouth, whilst on the upper 

 surface they are arranged in such a way that each plate carries one tubercle. On 

 the interambulacral areas there are at least 14 rows of tubercles of the same kind, 

 which are, however, reduced to two near the mouth, but continue to the apex, 

 where they become very small. 



Locality and stratigraphical position. — D^s Valley, horizon 4, 

 Remarks. — Owing to the thinness of the test, all the specimens are more 

 or less damaged or crushed ; this has, however, the advantage that the internal 

 side of the test can be examined, which shows that no internal septa exist, and the 

 specimen under examin ation really belongs to the genus Holectypm, not to DiS' 

 coidea ; apart from this, thelarge size of the periproct, rather speaks for Holectypus 

 than for Discoidea. 



This form is distinguished from all other known species by its ornamentation 

 of the test. 



Suborder: ATELOSTOMATA. 



Family: CASSlBULID^. 

 Subfamily : ECHINONEIN^. 

 Genus: PTRINA, Desmoul. 



Ptrina ataxensis, Cotteau. PI. III. fig. 4-4c, 4A., PI. IV. fig. 1-la, 2.2b, 



3.3a, 4.4A, 5-5b. 



IS63. Pyrina ataxensis, Cotteau; Echinides fossilea des Pyrenees, pi, 42, 



Dimensions of the different varieties. 



The shape of the test varies considerably as will be seen from the above figures ; 

 the larger number of specimens which have come under examination have a 

 pentagonal shape, slightly enlarged before and slightly contracted behind ; the upper 

 surface is inflated, and the profile, forming half an ellipse, slopes equally in either 

 direction ; this variety might be called var. pentagonalis^ 



In the next variety, var. tumida, the pentagonal shape is still preserved, 

 although not in such a marked way as in the former, the circumference having 



D 2 



