488 MR. HERBERT L. HAWKINS ON 



merely a retarded series of Irregular Echinoids, and some of the 

 orders of that subclass early became difierentiated from the 

 Pygasteridte by a relatively accelerated evolution. It is becoming 

 increasingly manifest that large groups of organisms, such as the 

 Irregular'^Echinoids, are not often homogenetic in the strict sense 

 of the word. When a series of forms that have been regarded as 

 belonging to an individual genus can be shown (as Beecher and 

 others have proved for some Brachiopoda) to pass through widely 

 divergent lines of ontogenetic (and therefore phylogenetic) 

 development, the problem of the evolution of a class or subclass 

 must be considered more complex still. Indeed, at first sight, it 

 would seem that, without the evidence of Ontogeny, no reliable 

 clue to genetic relationship can be deduced from even the most 

 accurate correspondence of adult characters. 



Stratigraphical palaeontology, however, shows a kind of extended 

 ontogeny which, although fragmentary, is infallible so far as it 

 can be understood. The same phenomena which complicate the 

 study of recapitulation in recent species are as widely developed 

 among the families and orders of past periods. Acceleration and 

 retardation, adaptation and degeneration, tend to obscure the true 

 sequence of genetic affinity to such a degree that, in the present 

 state of knowledge, only the bare outlines of the evolution of the 

 larger groups can be indicated. 



In this section of the paper, an attempt is made to show the 

 affinities (with persistent regard to stratigraphical relations) 

 which appear to link certain genera of the Holectypoida with 

 those of other orders. Little account is taken of the subsequent 

 changes which may have been developed in these other groups, 

 and no opinion is expressed as to their absolutely homogenetic 

 characters. The name of a fairly primitive member of each main 

 group is inserted in the diagram (text-fig. 60, p. 493) in its true 

 stratigraphical position, and by a thin vertical line each of these 

 names is connected with that of a characteristic genus now 

 living, which is usually regarded as belonging to the same group. 



1. P yg aster and G aler op ygtfjS. 



Galeropygus a,ppears in the Upper Lias with at least two 

 species, one of which {G. dumortieri Paris) is British. The genus 

 is thus contemporaneous with Pygaster sens. str. Gregory (50), 

 probably on account of its obviously primitive characters, included 

 it among his Pygasterida;, although in almost every feature it 

 ofiers a violent contrast to the diagnosis of that family. Practi- 

 cally the only diagnostic character in which it resembles an 

 Holectypoid is the apetaloid nature of its ambulacra. A feature 

 which would tend to connect it with some of the later Pygasters 

 (e. g., Macropygus trimcaUis) is the shape of the test, which is 

 commonly rather broader than long. The deep anal sulcus finds 

 a shallow counterpart in the posterior interradius of Pygaster 

 semisulcatus, but I have indicated above (p. 465) that this sulcus 



