494 MR. HERBERT L. HAWKINS ON 



those described by de Loriol (35), they also would show an 

 intermediate character. The presence of " bourrelets " round 

 the peristome in this genus are the only features that seem 

 antagonistic to its being regarded as ancestral to the Chjpca-ster- 

 series. 



The similarity between the small species of Dlscoidea and 

 Echiiiocyam.us caused Gregory (50), when revising the unnecessary 

 generic division of " Echinites " made by Duncan (44), to propose 

 the name Frotocyamus. The name is inadmissible on systematic 

 grounds, but would be morphologically appropriate. Echino- 

 cjjamus occurs first in the Upper Cretaceous, and has developed 

 bat few changes in structure from that time to the present day. 

 H. L. Clark (64) has recently suggested that the characters of 

 Echinocyamus are not primitive, but rather degenerate. On the 

 stratigraphical evidence I incline to regard them as truly 

 primitive, and to have retained ancestral traits by the retardation 

 of development consequent on their small size. 



Echinocyamus (of the Fibulariidse) was then directly evolved 

 from the smaller (typical) Discoidece, while Conoclypeits (of the 

 Clypeastridse) appeai-ed at the same period as a descendant of 

 (probably) the larger species of Dlscoidea (the " Pithodia " of 

 Pomel, 37). The former group underwent little change in 

 subsequent periods, but the latter became i-apidly differentiated 

 into the numerous and complex types that characterize the 

 other families of the Olypeastroida. 



VII. Summary. 



The Holectypoida are i-estored to the rank of an order of 

 the Echinoidea Irregularia. A classification, somewhat modified 

 from that proposed by Gregory (50), is given, and revised 

 diagnoses of the families, subfamilies, and genera are drawn up. 

 A comparative study of the morphology of the skeletal structures 

 of typical genera of the group is given ; and, in the light of the 

 results of this study, the course of evolution both within and 

 beyond the limits of the order is indicated. 



The Holectypoida are regarded as an annectant group of the 

 Irregular Echinoids, whose characters retain a considera.ble 

 uniformity owing to a persistent retardation of evolution. At 

 various periods ofltshoots from the Holectj-poid stock appeared, 

 which, usually with a relatively accelerated differentiation, 

 developed into the various orders and families of the Irregularia. 

 The order commenced in the Liassic period, and became extinct 

 at the end of the Cretaceous. Two of the groups of Echinoids 

 now living retain many features that were characteristic of the 

 Holectypoida (the Echinoneidse and the Fibulariidse). The other 

 groups of Irregular Echinoids show a much greater departure 

 from the primitive character, but they all possess some features 

 which indicate their Holectypoid ancestry. 



