104 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the first pinnule projected upon them. But no matter how much modification of 

 the following brachials there may be, a close examination soon reveals the fact that 



there are very impor- 

 tant differences between 

 them and the brachials 

 of the first pair. 



Homologies of the division 

 series of two ossicles. 



In all the ways in 

 which the first two 

 brachials differ from 

 the brachials succeed- 

 ing them they agree 

 with the elements of the 

 division series preced- 

 ing when these are com- 

 posed of two ossicles 

 only. This correspond- 

 ence is emphasized in 

 such species as Amphi- 

 metra ensifer (part 1, 

 fig. 86, p. 141), Pero- 

 metra afra, and Pero- 

 metra diomedecR (fig- 

 119, p. 79, and part 1, 

 fig. 112, p. 179), in 

 which the apposed ful- 

 cra of the synarthrial 

 articulation are greatly 

 produced, forming a 

 high conical tubercle, 

 which is repeated on 

 the line of union of all 

 the pairs in the division 

 series; the species of 

 Trofiometra (fig- 116, 

 p. 79, and part 1. fig. 88, 

 p. 145) in which the first 

 two brachials are dis- 

 proportionately large, 

 and the elements of the 

 division series are disproportionately large also; the species of Ste-phanometra 

 (figs. 125, 130, p. 79, Cenometra (part 1, fig. 87, p. 143), and Cyllometra, in 

 which the first and second brachials each bear a broad rounded ventrolateral 



Fig. 186. — Later.u. view of specimen op Neocomatella pdlchella. 



