26 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



Fig. 10.— a. glaculis, ninth gill of no. 8. 



Three OF THE AXES ARE CUT OFF. X15. 



The gills are best studied in No. 8. The most fully expanded gill of 

 this specimen is shown in fig. 10. It consists of nine or ten axes, 

 which arise from a short, curved, common basal structure situated 

 immediately posterior to the notopodium. The largest axis (see fig. 

 10, C) bears five branches, each of Vvdiich divides once, i. e., forks. 

 The longest of the resultant gill filaments are thumb or finger shaped 



structures, not more than about 0.7 

 mm. in length, and the shortest are 

 mere tubercles. This axis, measured 

 from its origin to the tip of the fila- 

 ments, is 2 mm. in length. The 

 smaller axes of this gill bear fewer 

 branches, only two or three, and these 

 may be simple, i. e., undivided, distally 

 (see fig. 10, A, B). Another gill, from 

 fragment No. 6, and one of the most 

 fully expanded gills present in the 

 whole of the specimens, is shown in 

 fig. 11. This gill is smaller than the 

 one just described. It is composed 

 of seven axes, the longest and largest 

 of which (A) bears three branches, each of which bifurcates, and one 

 of the two so formed again divides, but the other does not. In this 

 way three groups, each of three gill filaments, are clustered at the 

 end of the axis. From the base of this axis to the tip of its filaments 

 is a distance of about 1 mm. In all the gills examined the branches 

 of the gill axis are not given 

 off right and left, but are 

 clustered at the end of the 

 axis, so that each axis and 

 its branches look almost like 

 a hand and fingers (see, for 

 instance, figs. 10, 11, B). This 

 character distinguishes the 

 gills of A. glacialis from 

 those of any other species, 

 and especially from those of 

 A. cristata, the only other 

 species in which there are 

 eleven pairs of gills, for in A. cristata the branches borne on the gill 

 axes are much more numerous and are regularly arranged along the 

 sides of each axis, forming a typically pinnate gill. The gills of A. 

 glacialis are also much smaller than those of A. cristata. The gill axes 

 of a specimen of the latter 110 mm. long are from 4 to 5 mm. in 

 length, compared with 1 to 2 mm. in examples of A. glacialis of about 

 the same size. 



Fig. 11.- 



-A. glacialis, third gill of no. 6. 



THE AXES ARE CUT OFF. X35. 



Three of 



