in some Fossil EcJiinoidea. 263 



(This analogy is supported by the presence of ' bourrelet '-like 

 structures in such a genus as Echinocardium.) 



As far as I have been able to examine the structures, and by 

 inference from the arrangement of the pores in forms whose sutures 

 were indistinguishable, I feel confident in asserting that all phyllodes 

 have the same structure, and the same method of attaining it. In 

 fact, wherever a departure is made from the uniserial arrangement of 

 the pores in Irregular Echinoids (even in the peculiarly aberrant 

 petaloid portion of the anterior ambulacrum in Heteraster oblongus), 

 the sequence becomes triserial. When this fact is compared with the 

 extreme differences in structure that Duncan has described in the 

 llegulares (as may be exemplified by a comparison of a compound 

 ambulacral of Hemipedina with one of Strong ylocentrotus), the corollary 

 seems obvious. 



The Holectypoida and those Atelostomata grouped as Asternata 

 (see Gregory, 1900, p. 320) form a homogeneous group which, what- 

 ever complexity of interrelation they may possess, must be traced 

 back to a common ancestry. The triserial arrangement of the 

 ambulacral structures lends further weight to the arguments given by 

 Bather (1909, p. 106) for the belief that this ancestry is to be sought 

 among the primitive Liassic or Triassic Diademoids. I hope to deal 

 in a future paper with the relations of the various lines of descent 

 which spring from the Holectypoid stock. This paper may be 

 regarded as a preliminary to that more detailed study. Consideration 

 of the other features, apart from ambulacral plating (which all seem 

 to tend towards the same conclusions), must be deferred to the 

 larger paper. 



In the Holectypoida themselves, there is little advance in the 

 tendency towards phyllode development until Conulus is reached in 

 the Lower Cretaceous. In that genus the pore-pairs near the peristome 

 become triserial, and an approximately hypophyllodal structure is 

 attained. The genus Pgrina of the Echinoneidse appears perhaps in 

 the Jurassic in the form once called Nucleopygus, but it attains its 

 chief importance in the Lower Cretaceous. To the embarrassing- 

 similarities between Pgrina and Conulus must be added the fact that 

 the same method of plate-crushing obtains in both genera. The same 

 remark applies to Echinoneus, so that the distinction between the 

 Holectypoida and Echinoneidae becomes wholly arbitrary, at least 

 among their Cretaceous types. .The recent discovery by Agassiz 

 (1909, p. 490) of teeth in a young Echinoneus, and the description of 

 the teeth of Conulus (Hawkins, 1911, p. 70), are further proofs of the 

 close relationship that exists between the two groups. 



Galeropygus, of which the earliest known species occurs in the 

 Upper Lias, may well be claimed as the ancestral form of all the non- 

 Holectypoid irregular Echinoids except the Clypeastroids. Erorn this 

 genus there appears to have been a double line of descent. In one 

 series the ambulacra become strongly petaloid, while a well-marked 

 floscelle develops on the adoral surface. This series includes such 

 characteristic genera as Clgpeus and Pggurus, and attained its essential 

 characters quite rapidly, Clgpeus in particular being abundantly 

 represented by characteristic species in the Inferior Oolite. The 



