496 Herbert L. Haivkins — Studies on the Echinoidea. 



An analysis of the above comparison shows one feature of great 

 significance. In practically every quality in which the two forms 

 differ Tygastrides proves to be the less advanced from the Regular 

 (or better, Pj^gasterid) condition. It is, like Plesiechinus, a Regular 

 Echinoid save for the (incomplete) posterior migration of the 

 peripi-oct. Thus, if it really is a young stage, it must be affiliated to 

 some species that is less remote from a primitively Holectypoid 

 condition than Schinoneus. At least, Pygastrides cannot be a stage 

 in the development of that genus. 



6. The Pkobable Adult of Pygastrides. 



One of the most obvious qualities in the structure of Pygastrides is 

 the position of the sphseridia. These are single, and placed in 

 considerable depressions on the perradial lines. This is in complete 

 contrast to their disposition in ^chinoneus (and probably in the 

 fossil Holectypoida), and in the Cassidulidse and Spatangidse. It 

 compares with their arrangement in Regular Echinoids, but such 

 comparison is vitiated by the undoubtedly Irregular affinities of 

 Pygastrides. The only groups of Irregular Echinoids which possess 

 single, perradially situate sphaeridia are the three Clypeastroid 

 families of the Fibulariidae, Laganidse, and Scutellidae. In them, 

 the sphseridia are deeply sunken, and in some cases entirely buried, 

 in the test-surface; the sphaeridial pits of Pygastrides are unusually 

 deep. Unless it can be shown that sphaeridia are capable of migration 

 during ontogeny (a most improbable occurrence), it must be assumed 

 that the adult stage of Pygastrides lias deeply sunken, perradial 

 sphseridia. These need not of necessity be single, since new 

 sphseridia might be developed with advancing age ; but among the 

 known adult Irregular Echinoids that have median sphseridia, they 

 seem to be always solitary. This evidence, then, limits the choice 

 of an adult for Pygastrides to three families of the Clypeastroida, and 

 I cannot imagine that it is deceptive. 



Turning now to the proportions of the proximal coronal plates: 

 their great size in comparison with the others is at once apparent. 

 They form a strong, broad border to the peristome, strikingly unlike 

 the condition prevalent among most Echinoids. The superficial 

 resemblance of this primordial cycle to the perignathic structures of 

 DisGoides has already been noted — it is doubtful whether a true 

 morphological correspondence exists. The elongated ambulacrals 

 are reminiscent of those adoral to the phyllodes in the Cassiduloida, 

 Avhile the irregularity of the interambulacrals enhances the 

 resemblance. Indeed, apart from the presence of a perignathic 

 girdle and the position of the sphasridia, the peristorsial parts of the 

 corona of Pygastrides might easily change into a h'uly Cassiduloid 

 pattern. The processes might be vestigial structures, destined for 

 resorption as in EcMnon'eus ., but the singleness of the sphseridia 

 seems a fatal bar to the maintenance of the comparison. 



Again, the proximal coronals of Pygastrides are closely similar to 

 those of Echinocyamus (PL XVII, Fig. 4), a small genus that retains 

 throughout life many primitive Cl3'peastroid features. For example, 



