454 MR. HERBERT L. HAWKINS ON 



adapical surface, have exactly the plating-characters of Conulus. 

 A')nhlypygus seems to represent the simplest form of the Tertiary- 

 Recent section of the " Oassidulidse " (see Hawkins, 66), which is 

 characterized by Echinolmnpas and its allies. The genus will 

 receive fuller consideration in Section VI. of the present paper. 



IV. Comparative Morphology. 



Throughout this part of the paper references are made chiefly 

 to the structures of the four common British representives of the 

 group {Pygaster, Holectypus, Discoidea, and Conulus). Only when 

 peculiar characters or important contrasts occur in the less 

 abundant or foreign genera is a description of them inserted. 

 I have followed this principle advisedly, because, as this work is 

 largely one of generalization, it seemed preferable to use forms 

 where plenty of material was available, rather than to run the 

 risk of laying too strong an emphasis on a feature which, 

 occurring in an uncommon type, might be an individual 

 peculiarity. 



A. The proportions of the Test. 

 1. The circumference. 



All the forms which are included in the Holectypoida have 

 typically a radially symmetrical outline around the ambitus. In 

 some of the earlier foi-ms, notably in varieties of Fygaster senii- 

 sulcatus, there is a tendency for the outline to be quite sharply 

 pentagonal by reason of the prominence of the ambulacra, but 

 outside the borders of Pyyaster sens. lat. this feature rarely 

 appears. It is perhaps worth noting, in this respect, that among 

 the markedly pentagonal forms in my collection of the species 

 just mentioned, there are many of quite small size. Although 

 thus apparently a constant feature throughout life iu some 

 individuals, the angularity seems not to represent any phylo- 

 genetic stage, but to be mei'ely an irregxilar, though frequent, 

 variation. 



Among the Pygasters, when any departure from radial sym- 

 metry is encountered, it is found to result from a shortening of 

 the antero-posterior diameter in proportion to the width of the test. 

 This effect is appreciable in P. {Megapygu,s) umbrella, but reaches 

 its extreme in P. (Macropygus) truncatu,s. It is in all probability 

 due to an interference with the growth of the plates of the posterior 

 interambulacrum by the great size of the periproct. The trun- 

 cation of outline is rarely found outside the genus, but in Desorella 

 and Galeropygus it reappears, often to an increased degree. 



The slight groove which, in P. semisttlcatus, passes from the 

 periproct to the posterior margin, is probably due to the same 

 cause as the shortening of the interradium. It is very interesting 

 and suggestive to find a trace of the sulcus in this genus, in view 

 of the fact that its presence is a notable feature in Galeropygus. 



