492 Herbert L. Hawkins — Studies on the Echinoidea. 



3. Is Pygastridbs an Holecttpoid? 



Ignoring the question discussed in section 4, it is a matter of 

 considerable interest and importance to ascertain the systematic 

 position which Pygastrides can occupy. There can be little doubt 

 that Loven was right in regarding the "genus" as one of the 

 " Echinoconidse ", i.e. Holectypoida. His association of it with 

 Pygaster, while still eminently reasonable, is not quite so inevitable. 

 It would be difficult to locate it in any of the families of the 

 Holectypoida as at present recognized — it combines certain characters 

 that occur in all families of the order, and possesses some that are 

 not found in any of them. 



The Holectypoid qualities may be thus summarized : — The 

 approximately radial symmetry of its outline ; the central, circular, 

 fairly large peristome with apparent "branchial incisions" and 

 a perignathic girdle (hence presumably a lantern) ; the adapically 

 situated periproct, apparently in contact with the apical system and 

 not located in a sulcus ; the non-petaloid podial pores ; the serial 

 arrangement of primary tubercles on both areas and their scrobiculate, 

 crenulate, and perforate nature. Such an assemblage of characters, 

 nearly all of a positive type, makes it impossible to regard Pygastrides 

 as belonging to any order of Irregular Echinoids but the Holectypoida. 



Nevertheless, there are certain features in Pygastrides which do 

 not agree with the Holectypoid diagnosis. The perignathic girdle 

 seems devoid of interradial elements ; the ambulacra show no trace 

 of "plate-crushing", particularly near the peristome, and their pores 

 are conjugate and dissimilar, perforating the plates near the adapicaV 

 margins; the sphseridia are single, deeply sunken, and perradial in 

 position ; the tubercles seem to be all primaries, and the granulation 

 is peculiar. It might reasonably be argued that these (chiefly 

 negative) differences are due to the small size of the specimen. But 

 in examples of Discoides dixoni and Conulus suhrotundus of scarcely 

 greater dimensions the full Holectypoid requirements are fulfilled. 



Pygastrides maj', then, be considered as representing an Holectypoid 

 in which certain features are lacking, and in which one set of 

 structures, the sphaeridia, is apparently abnormally situated. The 

 disposition of the sphseridia in the fossil Holectypoida is not yet 

 known with certainty ; they were presumably not deeply sunken ; 

 but in Echinonens they occur on the ambulacral plates, superficially 

 placed, and to the number of three or four in each area. 



In an attempt to trace the affinity of Pygastrides with any of the 

 families of the Holectypoida less definite evidence appears. To the 

 Pygasteridse (and especially to Plesiechinus) it is similar in symmetry, 

 peristome (save for the sinallness of the branchial incisions), 

 perignathic processes (apart from their angle of setting and the lack 

 of buttresses), periproct, and tuberculation. The simplicity of its 

 ambulacral plating, though too complete, is another link with this 

 primitive family. Pygastrides resembles the Discoidiinse in the 

 relatively large size of the primordial coronal plates and in the 

 feeble development of the branchial incisions. The peculiar structure 

 of the interambulacra near the peristome is extraordinarily like that 

 contiguous to the "false ridges" of Discoides. 



