494 Herbert L. Hawkins — Studies on the Echinoidea. 



As Loven admitted, it is difficult to derive satisfactory evidence of 

 tlie age of Pygastrides owing to the absence of the apical system, but 

 to my mind the indirect evidence that indicates its immaturity is 

 conclusive. 



In point of size P. relictus is far smaller than any known adult 

 Echinoid, fossil or recent. Loven (Etudes, p. 79), regarding it as 

 a Cretaceous survival, remarks, "La plupart des formes cretacees 

 retrouvees vivantes a de grandes profondeurs sent comparativement 

 petites." But Pygastrides is not "comparativement", it is 

 " absolument petit". In the paper devoted to this problematical 

 Echinoid Loven admits that "Littleness . . . seems to be another 

 character by which it departs . . . from the recent forms among 

 which it lives ". The minute size, though not conclusive, is strong 

 presumptive evidence for youth. 



In the absence of definite measurements of the thickness of the 

 test, it is difficult to frame an opinion on the value of Loven's 

 description "rather thick". There is no evidence that young 

 Echinoids of as much as 3'5 ram. diameter are so little calcified as to 

 undergo shrivelling when dried — a quality that Loven apparently 

 would have expected in Pygastrides were it truly neanic. J£cMnoneus 

 cyclostomus seems "to have a perfectly rigid corona when it has 

 attained comparable dimensions, and certainly Parechinus miliaris is 

 quite massive at that size. 



Since P. relictus is an Irregular Echinoid, the tubercles, which 

 Loven cites as showing no specially youthful characters, are 

 certainly disproportionately large, and remarkably sparse. They 

 compare well with those of Ahatiis cavernosus (see PI. XVII, Eig. 5), 

 25 mm. in diameter, as figured by Mortensen (Schwed. Sudpolar. 

 Exped.). They are considerably larger in proportion to the size of 

 the test than in any adult Holectypoid, and even than in examples 

 of Plesiechinus ornatus of less than 8 mm. in diameter. Since the 

 tubercles of the Holectypoida are larger than those of other Irregular 

 orders (excepting the specialized ones in some Heart Urchins), and 

 they are constantly of fair dimensions in the relatively more 

 primitive Regular Echinoids, their large proportions in Pygastrides 

 certainly suggest that youthfulness is the cause. 



So little is known of the early post-larval stages of many groups 

 of Echinoids that the character of the " epistromal protuberances" 

 can hardly be used as a criterion of age. There are plenty of 

 granules and glassy tubercles on Eohinoneiis when the diameter is 

 only 4 mm. 



Reference to the ambulacral plates of P. relictus seems to me to 

 afford more indication of youth than of maturity. It is true that, 

 with the exception of the primordial plates of the columns, the 

 ambulacrals are all "Cidaroid" in character; but they are not lower 

 than those of PJchinoneus at 3-7 mm. diameter. Moreover, if 

 Pygastrides is an Holectypoid, and an adult one, it is unique in the 

 order in having no suspicion of "plate-crushing". In the small 

 gnathostomatous Echinoneus of about the same size there are already 

 demi'plates in the ambulacra, and in specimens of Plesiechinus 

 ornatus not exceeding 5 mm. in diameter some of the plates are 



