490 MR. HERBERT L. HAWKINS ON 



a Conul'us should possess vestiges of jaws, and a peculiar type of 

 perignathic girdle; but jaws are very rarely preserved in fossil 

 Echinoids, and Pyrina has a somewhat similar series of structures 

 around the peristome. The similarities between the two genei-a 

 include the shape (generally), the oblique peristome, the position 

 and shape of the periproct, the structure and arrangement of 

 the tubercles, the ambulacral plating, and the composition of the 

 apical system. Added to these there would probably be the 

 presence of vestigial jaws in young individuals of Pyrina, since 

 these structures have been found in a small specimen of the moi'e 

 highly specialized Echinoneus. An additional difficulty in the 

 separation of species belonging to the two genera results from the 

 fact that both were evolved at about the same time, and flourished 

 side by side during the Cretaceous period. 



So many correspondences in important structures cannot point 

 otherwise than to a close genetic affinity between Pyrina and 

 Conulus, and the only feature that can be considered to exclude 

 the former genus fi'om the Holectypoida is the absence of jaws in 

 the adult state. The presence of these organs in young specimens 

 cannot be considered sufficient evidence for the inclusion of 

 Pyrina in the order ; for, if vestigial characters are taken into 

 account in classification, by analogy the Mammalia, by reason of 

 their embryonic gills, would have to be classed with the Pisces. 



The earliest members of the Echinoneidse, such as NucleojyygvjS, 

 have the peiiproct in a supra-marginal position similar to that of 

 Anorihofygus. As Loven (36) has shown, a gradual migration of 

 the periproct takes place in this family along an exactly parallel 

 line to that passed through in the Holectypoida, until in Echinoneus 

 the anus is in a position similar to that of Discoidea. The features 

 of Desorella are so little known that it is unsafe to ascribe 

 a definite systematic position to it, and it has been ignored for the 

 purposes of the present work. I have regarded the Echinoneidse 

 as offshoots from Pygaster [Afact'opygus) in Upper Jurassic times, 

 which for some distance followed the AnortJiopygus branch, and 

 left it simultaneously with the Conulidee. They were at first 

 distinguished from that family by the accelerated degeneration 

 of their jaw-structures. 



3. AnortJiopygus and the Spatangidse. 



The earliest members of the Spatangidce proper appear in the 

 Lower Cretaceous. They are not very clearly distinguishable 

 from some other groups, especially from the Echinocorythidfe. 

 The structure of the apical system is, however, difi'erent in these 

 two families. The system of the Echinocorythidse is elongate, and 

 has been compared with that of the Collyritidae. (I have recently 

 shown (Hawkins, 70) that this structui-e could easily be evolved 

 from the Conulid type by acceleration.) The Spatangidse have 

 a compact apical system, which often nearly resembles that of the 

 Conulidfe, but is sometimes ethmolysian — that is, with the 



