108 



BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



Hontoloffifs of the division scries of four ossicles. 



A study of the species in which the division series are 4(3+4) shows that in 

 these division series the first two elements are obviously strictly comparable with 

 the first pair of brachials on the free undivided arms. Where special marks which 

 serve to identify the elements exist, as in the distinctive ornamentation in Oceano- 

 metra gigantea (fig. 220, p. 168) , 0. maffna (fig. 221. p. 171) , and 0. annandadei (fig. 

 222, p. 173), in other species of Thalassometridae, and in certain forms in the Char- 

 itometridse (figs. 679, 680, 681, p. 338), it is equally certain that the two outer ele- 

 ments in the division 

 series of 4(3+4) are 

 strictly comparable with 

 the first syzygial pair of 

 the free undivided arms. 

 Thus whereas the 

 division series of two os- 

 sicles are reduplications 

 of the first two brachials 

 of the free undivided 

 arms, the division series 

 of four ossicles are simi- 

 larly reduplications of 

 the first four brachials 

 of the free undivided 

 arms. 



Since the distal ele- 

 ment of the first brachial 

 pair and of all the re- 

 duplications of this pair 

 situated proximal to it 

 always bear two deriva- 

 tives on the outer border, 

 two arms, two division 

 series, a division series and an arm, an arm and a pinnule, or, abnormally, two pin- 

 nules, it is easy to see why in division series of 4(3+4) the second element always 

 bears a pinnule. 



Occasionally division series of four ossicles occur in which the elements of both 

 pairs are similarly united by synarthry. These are unusual and abnormal, but 

 they have been reported in Cyllometra albopurpurea (" C. anomala"), in Hetero- 

 metra quinduplicava ("Antedon anceps"), and in Adelometra angustiradia. They 

 represent two division series of two ossicles each, in which the outer element of the 

 first bears a pinnule instead of a second arm. 



In-egnlarlHes in the occurrence of the division series of 4(3 + J,t. 



Division series of 4(3+4) appear to be correlated with a very broad and patent 

 habit, exceptionally reduced raclials, and an exceptionally large visceral mass, the 



FIG. 188. CENTRAL PORTION AND ONE POST-RADIAL SERIES OF SPECIMEN OF 

 COMANTHEEIA ALTERNANS FItOM PHILIPPINE ISLANDS. 



