SHA-UnCUINS. 



469 



degree. As regivrds Conulus, only the Echinoneidfe, among its 

 external I'elatives, retained the apetaloid character. The 

 Echinolampidte and the Conulopsis group show a pronounced 

 subpetiiloid development. Nevertheless, the fact remains that, in 

 the matter of their ambulacral pore-structure, the later geneia of 

 the order conform more absolutely to the letter of the diagnosis 

 than the earlier forms. 



Text-fig. 57. 



F 







H 



:; . 



* • 



• » 



Diagram showing some characteristic plates of the ambulacra (from the adapical 

 surface) in A. Fygaster sens. str. ; B. Galeropi/gus ; C. Megapygus \ 

 D. Pileus; E. Discoidea; P. Conoelypens; G. Conulus; H. Pyrina. 



The pore-pairs in Pileus show an anomalous character in being 

 biserial on the adapical surface, without any corresponding inter- 

 fei'ence with the primary nature of the ambulacral plates. There 

 seems nothing among Irregular Echinoids to compare with such 

 a condition, which recalls the similarly inverted development of 

 biserial pore-pairs in the ambulacra of JJiplopodia. The somewhat 

 analogous appearance of the pores in the anterior ambulacrum of 

 the Spatangoid Ueteraster is accompanied by " plate-crushing " in 

 the structure of the area. Pileios, in this respect, as in some 



