44 



HAY. 



[Vol. II. 



The pectoral fins have been described by Professor Cope and 

 Dr. Crook (Cope, Cirt. Vert., pp. i86, 193, 204; Crook, 

 PalaeojitograpJiica, vol. xxxix, p. 1 19). Neither of these authors 

 compares the fin structure with that of other fishes, although 

 a community of structure is perhaps implied. The large saber- 

 shaped spines, each consisting of an upper and a lower half, are 

 remarkable enough ; but when comparison is made with the fins 

 of a shad or of a tarpon the arrangement of all the parts is 

 easily comprehended. The first pectoral ray of Xiphactinus 

 resembles quite closely that of Tarpon. It differed in being, 

 relatively to the size of its owner, somewhat, but not enormously, 

 larger. It differed further in having lost, apparently to the very 

 tip of the ray, the cross-segmentation. In Tarpon this persists 

 in the distal half of the ray. Doubtless, the spine-like rays of 

 Xiphactinus were not so flat as they are now presented to us. 

 It is quite probable that the rays succeeding the first one were, 

 : toward their distal extrem- 



ities, not only cross- 

 segmented, but also longitu- 

 dinally split, as in other 

 fishes. 



Professor Cope (^'/r/. Vert., 

 p. 186) has described the 

 ventral fins and their sup- 

 porting bones. The latter, 

 pelvic actinosts, usually 

 termed the pelvic bones, are 

 called by Professor Cope the 

 femora. He also figures them 

 (p. 192, Fig. 9, and PI. XLV, 

 Figs. 7, Ja). I possess a 

 well-preserved specimen of 

 the pelvis and the ventral 

 fins of X. tJiaiimas, and from 

 these it becomes evident 

 that the pelvis figured by Professor Cope was very defective. 

 This may be seen by comparing the figures above referred to 

 with my Fig. 10, which represents the pelvic actinosts seen 



Fig. 10. — X. thauinas. Pelvic bones and base 

 of fin. X J. 



