50 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



platyacanthum. The most evident distinctions are the slenderer rays and more 

 delicate structure generally of the southern form. The third or lowermost row of 

 actinolateral plates, even in the very long rayed type-specimen, extend only a third 

 the length of the ray, and fully half of these plates are inconspicuous, being very 

 largely covered by plates of the second row. In a comparable specimen of sac- 

 culatum from the type-locality which has been cleaned and dried the third series 

 extends five-eighths the length of the ray or about twice as far as in ectenes, and there 

 is a very rudimentary fourth series, also, for a short distance beyond the interbrachial 

 angle. The abactinal skeleton is much less rigid in very large specimens (but in 

 medium-sized examples the difference is negligible), the adradial plates being smaller 

 and more loosely articulated with the relatively slightly smaller superomarginal and 

 carinal plates. The adradial papular areas contain proximally two to four papulae, 

 mostly the higher number. The abactinal pedicellariae are conspicuous, 2 to 2.75 

 mm. long, and are sometimes longer than the conical spinules, which are relatively 

 to their length much thicker than in medium-sized examples. In the latter the 

 pedicellariae are much thicker than the spinules, and these are in turn much slenderer 

 than in medium-sized examples of true sacculatum. In other words, the spine differ- 

 ences are more marked in smaller specimens. In the type the lateral spines are 

 decidedly flattened and the longest are about 5 to 6 mm. They do not occur on 

 every plate, certain plates being skipped (as in sacculatum). The spines have a 

 thick sheath. On the abactinal surface the spines, pedicellariae, and papulae are so 

 nearly of the same length that the surface seems to be covered with nearly uniform 

 close-set papillae. 



The pedicellaria on the furrow spine reaches a length of 3 mm., has unequal jaws, 

 the ends being bent so as to greatly resemble a miniature bird's head with an unusually 

 large ibis-like beak. 



Young. — There is a very complete series of young mostly from station 4425, 

 ranging from R 11 mm. (PI. 25, figs. 5-12.) The most conspicuous feature is the 

 relatively huge terminal plate, of a rather broadly oval contour, deeply notched adcen- 

 trally, and bearing on the ambitus eight radiating sharp relatively long spines (usually 

 broken). This plate is 3 mm. long (R 1 1 mm.) or one-third the length of ray measured 

 on the side, and 2.25 mm. in diameter; the longest terminal spine is about 2 mm. 

 Relative to length of ray, the marginal and carinal spines are much longer than in 

 the adult, and relatively large pedicellariae are already numerous. There is one 

 actinolateral series of plates. The adradial plates are largely covered by the two 

 adjacent series. -The adambulacral plates are not yet differentiated into two sorts, 

 and they carry a transverse comb of four or five relatively long spinules, the series 

 alongside of the furrow reminding one of the subambulacral combs of Solaster. Tube- 

 feet in two series and with relatively much larger disks than in adults. 



In a specimen with R only 18 mm., differentiation of adambulacral plates is 

 progressing, and relatively large pedicellariae have appeared on the furrow spines. 

 A second series of actinolaterals is present and the numerous abactinal pedicellariae 

 are relatively very large. 



An example of the size of R 80 mm. is notable for its numerous large pedicellariae, 

 relatively much slenderer spines than in typical sacculatum, and for already having 



