1018 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [38] 



articulating with its basal portion. In Geratodus they are the biserial 

 cartilaginous axis of the limb. In Polypterus they are the anterior and 

 posterior basal elements of the limb with intercalated elements inter- 

 vening. In Chondrostei they are the segmented cartilaginous basal 

 plate which subdivides distally into six bars.* In Elasmohranchii the 

 axial plate of the paired fins is more comi)lex, and usually consists of 

 three well-marked subdivisions arbitrarily named pro- meso- and meta- 

 pterygium,t external to which there is a greater or less number of seg- 

 ments resting upon the preceding three pieces, with a more or less par- 

 allel course, though often diverging and dividing, as in the Eays, for 

 example. The Torpedo has a so-called cephalic fin resting in part upon 

 antorbital processes and partly on the cranial rostrum. These .are the 

 principal forms of the axial skeleton of the paired fiins. They are the 

 most ]}rimitive and embryonic representatives of the limb skeleton found 

 amongst Vertebrates, as embryological research has shown. 



In the Teleosts, where great specialization and even loss or shifting, 

 of the posterior fins have taken place in some forms, still greater reduc- 

 tion has occurred. The axial| basal elements of the pectoral may be 

 present as a single plate {Gastrostomus), or, at most, be represented by 

 a few short (usually three or four) actinophores, often supporting a 

 greater number of rays. The group Actinopteri of Cope {Teleostei of 

 this paper) is thus defined by its author: "Derivative radii few in the 

 fore-limb, sessile on the scapula; wanting or very few and rudimental 

 on the hind limb, so that the dermal radii rest on the axial element." 

 It is therefore evident that the Teleosts represent the extremest term 

 of specialization attained by the limb-skeleton in, fishes, and that in the 

 pelvic limb, at least, the whole limb is sometimes represented by true 

 rays only. In all of the Lyrifera the axial sl<eleton of the paired Jim is 

 developed from the middle or medullary part of the mesohlast, ivJiich is- 

 thrust out into the primitive epidermic or epiblastic lateral fi n -f ol d s, jnst 

 as in the case of the unpaired fins, but there is not that palpable con- 

 tinuity of the skeletogeuous tract of the paired fins with that investing 

 the chorda, as in the case of the same tract in the unpaired fins. 



The next point of great morphological and theoretical importance in 

 the definition of the true rays is their primordial 'relation to the axial 

 skeleton of the fins. We found that the latter originated in the central 

 parts of the mesoblastic substance of the embryonic fin, whereas the 

 true rays we find to originate in the superficial j)art of the same layer, 



* Vide Giinther: Memoir on Geratodus. Philos. Trans., II, 187t, p. 533. 



t Vide Huxley : Anatomy of Vertebrated Animals, p. 38. 



t The term axial elements is here used to signify the endoskeleton of the whole of 

 the mobile portion of the limb, and not to signify the endoskeleton which forms the 

 X)rosimal elements or segments of the pectoral and pelvic limbs of fishes, as imjjlied 

 by Cope in the phrase "'axial series" in his paper in Proc. Am. Philos. Soc, May, 1877, 

 and Report of State Commissioner of Fisheries of Pennsylvania for 1879 and 1880, 

 pp. 67, 68. This explanation has been thought desirable in case the reader might be 

 led to compare this memoir with, those by Cope. 



