220 Professor H. L. Hawkins — 



between the two genera ; while the correspondence in detail seems 

 too close and too comprehensive to be merely fortuitous. Remove 

 the interambulacral tubercles from the adoral surface of 

 Pseudojoygaster, and the resulting aspect is almost exactl}' that of 

 Galeropygus. Even the ambital outline, with the more prominent 

 angles on the interambulacra, is in agreement ; the straight, narrow 

 ambulacra, with suggestion of slight orad expansion into incomplete 

 hypophyllodes surrounding a faintly eccentric, proportionately 

 minute peristome could hardly be more alike. The very slight 

 degree of asymmetry in the peristomial aperture, and the lack of 

 dental armature are additional resemblances ; but, in view of the- 

 preservation of Pseiidopygaster, must not be unduly emphasized. 

 Further, I am of opinion that complete obliteration of the inter- 

 radial tubercle-series of Pseudojnjgaster would weaken the com- 

 parison. On the adoral surface of Galeropygi/s two bkmt carinae, 

 reinforced by knobs situated on or near the transverse sutures, 

 pass up the interambulacral columns to disappear at the ambitus (see 

 Fig. 9). The transverse sutures sway adorally in crossing these 

 carinae, which may well be considered as relics of the abandoned 

 tubercle rows. Pseudopygaster shows approximate atrophy of the 

 tubercles above the ambitus ; and, while it is possible to ascribe some 

 adaptive purpose to this phenomenon, the " law of localized stages 

 in development " would suggest that subsequent derivations from 

 Pseudojoygaster would show progressive decrease in major tubercula- 

 tion. Again, the sporadic occurrence of scrobiculate granules on 

 the adapical surface of Pseudopygaster would produce, by an increase 

 in their number relative to the normal granules, an ornamentation 

 precisely like that of Galerojjygus. 



In the matter of the periproct an element of contrast appears. 

 That aperture in Pseudopygaster is broad and extends to the 

 ambitus ; in Galeropygus it is narrow and rarely covers half the 

 distance between the apex and posterior margin. The deep posterior 

 sulcus of Galeropygus seems foreshadowed by the shallow invagina- 

 tion around the exposed parts in Pseudopygaster ; and its depth in 

 the former genus is probably connected with the depressed shape of 

 the test. The apical system shows marked differences, assuming 

 that its dimensions in Pseudopygaster are correctly determined ; 

 for that structure in Galerojyygus is exceptionally small in area. 

 However, a morphogenetic sequence back from Galeropygus would 

 almost certainly imply a tendency towards Cidaroid qualities, and 

 would thus involve increase in the relative dimensions of the apical 

 system. 



It thus appears that Pseudopygaster shows closer correspondence 

 with Galeropygus than with any other Irregular Echinoid, although 

 its affinities, Irregularity apart, are much more with the primitive 

 Orthopsidae such as Mesodiadema and Loriolella. At least, it 

 has only the remotest connection with the Pygasteridae. It 

 is, of course, possible to dismiss this new type as a blind 



