Herbert L. Hawkins — Studies on the Echinoidea. 493 



primordial interarabulacral plates block the path of the advancing 

 columns, giving a type of "plate-crushing" comparable with that 

 described by me in Lovenia forhesi. The minor ornament of the test 

 is suggestive of some of the guttate granules of Holectypus depressus, 

 and perhaps of the glossy granulation of the adoral surface of Conulus. 



Thus, while showing a preponderance of Pygasterid characters, 

 Pygastrides exhibits important differences from the known members 

 of that family, combining in the one small corona qualities that are 

 shared among all the families of the Holectypoida, and especially 

 suggesting some affinity with the Discoidiinse. It may be defined as 

 a generalized Holectypoid with mainly primitive traits. 



Finally, there is one feature in Avhich Pygastrides probably differs 

 from the normal Holectypoida and is certainly different from their 

 nearest living representatives. It is a deep-water form. Although 

 it is difficult to speak with certainty on the bathymetric range of 

 fossil types, all the evidence seems to show that the Pygasteridse 

 (to which Pygastrides makes the closest approximation) were 

 essentially denizens of shallow water. The latest Pygasterids, snch 

 as Anorthopygus, are only known from the littoral facies of the ChaJk- 

 sea deposits (e.g. the Hibernian Greensand and the Haldon Hill 

 remanie), never being found in the regions of open-water ooze. 

 Eclmioneus abounds chiefly between tide-marks, and Micropetalon is 

 not known from a greater depth than 24 fathoms. Possibly some of 

 the Cretaceous Holectypoida, such as Conulus albogalerus^ may have 

 inhabited water of considerable depth, but there is no proof that they 

 lived at such a depth as 250 fathoms. Moreover, they are morpho- 

 logically- the least like Pygastrides of any members of the order. 

 This point, though of little systematic value by itself, is worth 

 noting in conjunction with the other divergences of Pygastrides from 

 the Holectypoid type. 



4. Is Py&astsides Adult, or a Post-Laeval Stage? 



It follows from the discussion in the preceding section that 

 Pygastrides, if it be a genuine genus, must find a place among the 

 Holectypoida, although the diagnosis of the Order would need some 

 modification for its inclusion. As at present known, it cannot 

 possibly be associated with any other order of Echinoids. 



Loven seems to have been convinced that this small specimen is 

 adult. He supported his contention by reference to the ''rather 

 thick" and rigid test, the character of the tubercles and epistromal 

 prominences, and the depressed ambulacrals. He further remarked 

 that, were it larger when adult, specimens would hardly have 

 escaped capture. This last contention may be dismissed at once, 

 since it is based upon the assumption that the adult form would 

 closely resemble the small specimen. It does not follow that, 

 supposing P. relictns to be an ontogenetic phase, the adult is not 

 already known, the correlation of the two being at present 

 impossible. Zoologists are painfully aware of the difficulty of 

 identifying a larval stage with its adult form, even when both are 

 fairly abundant. It is reasonable to suppose that, if P. relictus is a 

 young stage, its adult equivalent has already been discovered. 



