Herbert L. HatuJcins — Studies on the Echinoidea. 495 



already distorted. Thus the extreme simplicity of the ambulacral 

 plating is again suggestive of youth. 



Other features in P. relictiis which are concordant with the view 

 that it is a young form are: (1) the great size of the proximal 

 coronal plates, (2) the general rotundity of the test, and (3) the 

 relatively large size of the sphseridia. 



The position of the periproct gives little help in this discussion. 

 Certainly, if Pygastrides were adult, it would prove to be the least 

 progressive of all known living Irregular Echinoids in this respect 

 (excluding perhaps '^ Nucleolites" recens), being no more advanced than 

 the Lower Jurassic Plesiecliinus. But even so advanced a ISpatangoid 

 as Ahatus cavernosus (PI. XVII, Fig. 5) has the periproct on the 

 adapical surface and in contact with the apical system when it has 

 attained a diameter of 2*5 mm., while at l'9mm. it is definitely 

 endocyclic. A less specialized Irregular Echinoid might well have 

 a " Plesiechinoid " periproct when it had reached the dimensions of 

 Pygastrides. 



As a result of the considerations put forward above, I am 

 convinced that Pygastrides relictus is an early post-larval form of 

 some larger species, the adult stage being, in all probability, some 

 type of Irregular Echinoid already known. 



5. CoMPAKisoJr OF Pygastrides with JSohinoxeus or the same size. 



For the purpose of this comparison I have relied upon "Westergren's 

 beautiful drawings (1911, Mem. Mus. Comp. Zool. Harvard, 

 vol. xxxix, No. 2, pi. xi). 



The corona of Pygastrides is more circular in outline and more 

 elevated tlian that of Echinoneus at 3"7 mm. diameter. 



The peristomes of the two forms are similar in proportion and. 

 shape, but in PJchinoneus the primordial coronal plates are considerably 

 reduced in size. There are three sphseridia in each ambulacrum in 

 Echinonetis, in contrast to the single one in Pygastrides. 



The perignathic girdle is in a far higher degree of development in 

 Pygastrides than in Echinonens, and the processes are only partly 

 based upon the ambulacral plates. 



The periproct of Echinonius has already reached its adoral situation 

 at this stage of development — a marked advance on its c&ndition in 

 Pygastrides. 



The ambulacra of EcM7ioneus have the primordial plates not 

 strikingly dissimilar from the rest, while in Pygastrides these plates 

 are very high. Demi-plates occur already in the former type, while 

 there seems to be no trace of disturbance in the regularity of the 

 shape of the ambulacrals in the latter. The pores of Echinoneus are 

 round, disjunct, and normally situated in oblique pairs on the more 

 adoral parts of the plates — the reverse is the case in Pygastrides. 



In the interambulacra a similar relation of the primordial unpaired 

 plates exists to that found in the ambulacra. 



The tuberculation of Echinoneus at 4*4 mm. is closely comparable 

 with that of Pygastrides. 



The young forms of Echiiioneus occur between tide-marks with the 

 adults, while Pygastrides was dredged from deep water. 



