218 Professor H. L. Hawkins — 



pentagonal than the holotype, but shows nothmg distinctive save 

 the peristome, which agrees absolutely with that of the more complete 

 specimen. 



(iii) COMPAKISONS AND DISCUSSION. 



There can be no question as to the approximation of Pseudo- 

 jpyqaster to Loriolella. Save for its absolute " Irregularity " it 

 shows few features of difference worthy of more than specific 

 separation. The apparent feebleness of the branchial incisions in 

 Pseudojiygaster might be considered an important contrast ; but in 

 view of the unsatisfactory preservation of the two specimens, it 

 seems unsafe to lay much stress on this point. The relationship 

 between the two genera can be expressed in the following way : 

 if fresh material of Pseudopygaster should prove that I am mis- 

 taken in my belief in the absence of the fifth genital plate, or if 

 new specimens of Loriolella should show that Lambert has wrongly 

 ascribed such a plate to that form, then the question of including 

 the new genus in Loriolella would be open to discussion, although 

 the species certainly cannot be merged in L. ludovici. I have no 

 anticipation of the former contingency, and should be surprised if 

 Lambert's statement proved incorrect, although 1 know of no 

 evidence in its support, Tt is an interesting commentary on current 

 classification that, in the light of present knowledge, these two genera 

 stand together so closely as to suggest unity, but between them 

 there is a " great gulf fixed ", by which they are destined to 

 remain in two different subclasses ! The anomaly is rendered more 

 startling by the conception that Pseudopygaster must surely have 

 been virtually a Loriolella in earlier ontogeny ! 



Although the general form and aspect (and, of course, the 

 periproct) of Pseudopygaster are in strong contrast with those of the 

 various species referred to Mesodiadema, many of the details of 

 the one are extremely like those of the others. The diagnoses of 

 Mesodiadema given by Bather (1909, " Triassic Ech. of Bakony ") 

 or Lambert (1911, loc. cit.), taken separately or combined, could be 

 applied almost verbatim to Pseudojjygaster, omitting the vvord 

 " Diadematoid " from the former and the phrase " Test de petite 

 taille " from the latter. Indeed, the inconspicuous character of 

 the branchial incisions in the Orthopsid genus brings it nearer to 

 Pseudopygaster than Loriolella, with its " scissures — assez pro- 

 fondes ". Perhaps the large size of the apical system in the last- 

 named types may counterbalance this feature ; since the apex of 

 Mesodiadema, though unknown in detail, seems not to have been very 

 extensive. Nevertheless, on the evidence available, its exocyclic 

 periproct is the only satisfactory peculiarity of Pseudopygaster, 

 when compared with Mesodiadema, although size and form provide a 

 superficial distinction. In this connexion it is interesting to 

 notice that the tubercles in Triassic species of Mesodiadema had 

 confluent scrobicules, and that the proportionate size of the 

 tubercles is much reduced in Liassic form (see Figs. 7, 8). 



