SKA- URCHINS. 



455 



and inosfc of the non-llolectypoid Jurassic Irreyulni- genera. A 

 further reference to this feature will ))e found in tlie .section on 

 the interambulacra. (p. 465). 



In Jlolecti/pus thei-e is a tendency opposite to that of Pyyaster^ 

 but one probal)ly caused by the same agent. In such a form as 

 II. depressus, where the periproct is of very large size and is 

 situated on the adoral surface, there is frequently a backward 

 [)rojection of the posterior interambulacrum to accommodate it. 

 Even with this projection, there is often but a thin rim of test 

 between the periproct and the peristome — a fact which shows the 

 liecessity for some such ari'angement. In many of the Jurassic 

 species which have a inarginal periproct, a similar tendency is seen 

 (e.g., Holecti/jJics oblo)i(/nsy\^i-ight). Although the lengthening of 



Text-fig. 54. 



])iagrams of tlio adoral surface iii some Holoctypoida and their allies showing 

 the shape of the ambitus and characters of the peristome. 



A. Ffigaster semisulcatus, B. Galeropygas agarwiformis. C. Pifgastcr (Macrn- 

 pi/gws) laganoides. D. Holecti/pv,s depresst(,s. E. Discoidea ci/lindrica. 

 ¥. A Clj'peastroid. G. Conulns albogalerug. H. A cretaceous Echiiioiioid, 



theautero-posterior axis results in a bilateral symmetry compar- 

 able with that of many of the Echinoids which are more advanced in 

 " Trregulaiity," it was not a feature retained by the Holectypoida, 

 after Jurassic times. All the species of the subgenus CdiiiJiolec- 

 tj/pns, and all of Discoidea, have an approximately cii-cidar outline. 

 The slightly indented character of the interradii on the ambitus 

 of iJifscoidea gives an alternating concavity and convexity to the 

 margin which may be compared with tliat of a Ch/peaster. In 

 Coniibis the tendency to elongation reappears, particularly in the 

 large, high-zonal foi'ms of fj. alhngaleras, ^v■here the marginal 



