Herbert L. Hawkins — Studies on the Echinoidea. 499 



JEchinoneus, but at least the muscle scars develop later in 

 JEchinocyamus. In the Clypeastridse, again, no interradial elements 

 occur, the protractor muscles finding attachment on the processes 

 themselves. 



The Fibulariidjfi and Laganidse are certainly less specialized in 

 most characters than the Clypeastridse. It is therefore strange to 

 find the fused interambulacral processes — a highly specialized 

 condition — occurring in the former groups, Avhile more typically 

 Holectypoid qualities are preserved in the latter. But if the 

 theoretical structure of the "auricle" of Ecliinocyamus given on 

 PL XYII, Fig. 7t', is in any <^'ay correct, tliat type of girdle is 

 more closely akin to the Holectypoid (especially to the Discoidiid) 

 type than the ridgeless girdle of Clypeader. The two types of 

 Clypeastroid girdle must, however, mark two distinct and divergent 

 lines of descent ; in one all structures become ultimately inter- 

 ambulacral, in the other ambulacral, in position. Both types differ 

 from the true Holectypoid, and it is premature to ascribe greater or 

 less specialization to either. 



8. The Beaeing of Pygastrides on Post-Holectypoid Phvlogent. 



It will be realized from the foregoing descriptions and arguments 

 that Pygastrides is in most respects an Holectypoid, in many a 

 Pygasterid, in some a Discoidiid, and is probably the young of 

 a primitive Clypeastroid. Of' the young Echinoneus of similar size 

 it may with equal j^ ' tice be said that it is in most respects 

 an Holectypoid, in many a Pyrinid (or Conulid), and in some 

 a Discoidiid. The young Alatus at a smaller size may be said to be 

 Pygasterid in apical and periproctal characters, Pyrinid in shape, but 

 already Spatangid in coronal plating. The last-named form is 

 evidently so accelerated in its ontogeny that no certain conclusions 

 can be drawn without more evidence. The two others are more 

 restrained in development, and their evidence on the phylogeny of 

 their adults is intelligible and conclusive. Of Echinoneus and its 

 relatives I hope to treat in the near future, but it will suffice here to 

 incorporate the evidence of Pygastrides into the scheme of post- 

 Holectypoid evolution. 



Palaeontologieal and morphological study link the Clypeastroida 

 inseparably with the Holectypoida. The peculiar distribution of 

 the madreporic pores and the development of internal supports 

 associate them more particularly with the Discoidiinse. Whether 

 Ecliinocyamus and its relatives be primitive or degenerate, they are 

 certainly the simplest of the Clypeastroida. Pygastrides, if the 

 above interpi'etation of its nature is correct, adds a conclusive 

 ontogenetic proof of tlie near alliance between the two orders, and 

 suggests that the Fibulariidse are truly primitive. Almost the only 

 features in which Pygastrides differs from a true Holectypoid consist 

 in the omission of certain structures, most of which are obsolescent 

 in the Holectypoida themselves. 



9. Summary. 

 Pygastrides relictus, Loven, is believed to be an early post-larval 

 stage in the development of some Irregular Ecliinoid. Reasons are 



