ISOMETRINAE 



177 



Isometra angustipinna (Carpenter) 



Antedon angustipinna Carpenter, 1888, p. 189, pi. xxix, figs. 1-4. 

 Isometra angustipinna (part) Clark, 1908, p. 133. 



The single specimen is small: Carpenter gives its spread as probably 5 cm. 



Only one cirrus remains attached to the centrodorsal, an upturned cirrus of 27 

 segments but of an immature appearance. There is one small, detached, cirrus of 22 

 segments which is complete and mature. The first two segments are short, the third is 

 longer than broad and flared at the end. The fourth 

 to about the eighth are longer than broad, but de- 

 creasing in length; their ends are flared, more 

 strongly on the dorsal than the ventral side but 

 sufficiently on the ventral side to make that edge of 

 the lower part of the cirrus uneven. The other seg- 

 ments are shorter, about as long as broad, with no 

 dorsal spine but a faint keel which becomes obsolete 

 on one or two segments before the opposing spine. 

 The opposing spine is strong, the terminal claw 

 strongly curved. 



The shapes of the primibranchs and lower 

 brachials are shown in Fig. 14 a; they have sharp, 

 straight side-edges. The syzygial pairs beyond the 

 third are separated by only one brachial. 



Pi is very short, about 2 mm., of five or six seg- 

 ments of which all, or nearly all, are attached to the 

 disk by a web of tissue. On some of the arms P2 is 

 of the same length as Pi and of about six segments, 

 but on most it is a genital pinnule about 5 mm. 

 long. The third and fourth segments of the genital 

 pinnules are more strongly and less symmetrically 

 expanded than those of /. lineata. They are longer 

 than broad and more strongly expanded on the distal than the proximal side (Fig. 14 b). 

 The gonads are fully developed and appear to be testes. 



The arms are broken off' shorter, and the pinnules which remain are in a much poorer 

 state of preservation, than those of /. lineata, so that, although an ambulacral skeleton 

 can be seen, it is difficult to be sure of its nature. There appear to be two sets of plates, 

 side- and cover-plates, one or two pairs on either side of each segment. One of each is 

 shown in Fig. 14c; the "side-plate" is rod-like with a root-like base; the "cover- 

 plate" with its fan-like distal portion, resembles the plates of /. lifieata. No spicules 

 can be seen in the tentacles. 



Fig. 14. Isometra angustipinna. a, centro- 

 dorsal and proximal part of a ray, x 8. 

 b, P4, a genital pinnule, x 12. c, a side- 

 and a cover-plate from a distal pinnule, 

 X 150. 



