362 PROCEEDINGS OF THE ACADEMY OF [1887. 



type of displacement of positions of a limb, cannot fairly be com- 

 pared with the case of the Physoclists, in which the peduncle of the 

 limb also shifts its position in reference to the origin of its nerve 

 supply. The foregoing view as to the origin of the n. collector, has, 

 I would state here, been arrived at independently by Wiedersheim 

 in the second edition of his Lehrbuch, (p. 323). 



It might also be added that, wherever the proplexus and post- 

 plexus are parts of a continuous series of pairs as in Raia, the fin- 

 folds of both pairs of limbs are also continuous at the time they first 

 grow out, whereas, when they are not continuous, and when the an- 

 terior and posterior limbs grow out as more or less widely separated 

 folds, but with abortive limb-buds intervening, which never form a 

 part of the permanent limb, as may be observed in the embryos of 

 Mustelus, the two plexuses are separated by an interval. 



Such forms as Raia, also exhibit extensive fusion of the proximal 

 ends of the separate radii, leading to the formation of the longest 

 possible type of pro- and metapterygium, whereas in the types like 

 Mustelus, the propterygium and metapterygium are composed of fewer 

 radii. They cannot, therefore, be homologous and we cannot on that 

 account take any metapterygium or j^ropterygium or whole fin of 

 any particular type, as the ground form from which an ideal arch- 

 ipterygium or cheiropterygium may be supposed to have arisen. It 

 is probably better for the present to assume, for the reason that an 

 inexact homology exists when the limbs of different vertebrates are 

 critically compared, that the different types of limbs, as we now see 

 them in the higher groups, have arisen independently and differently 

 in the different phyla. That the manus and pes, as seen in the va- 

 rious vertebrates, from Amphibians upward, show a common plan 

 there is no doubt, but of the fact that similar structures may orig- 

 inate independently of each other, we have numerous instances in the 

 animal kingdom. 



It is not even certain that there may not be more than two ])airs 

 of limbs developed in certain Fishes. In the cephalic fins of Torpedo- 

 and Narcine, the radii rest upon the rostral cartilage of the cranium, 

 thus separating them from the shoulder girdle. This attachment is 

 supposed by Gegenbaur to be a secondary one, the radii of the ceph- 

 alic fins being part of the radii of the propterygium, secondarily de- 

 detached from tlie anterior portion of the pectoral. But for this 

 opinion there do not seem to be more valid grounds, than for the 

 su])position that the cephalic fins are outgrowths of the head, since 



