358 PROCEEDINGS OF THE ACADEMY OF [1887. 



fused by fours into eight, this brings the first pair of the post-plexus 

 into the position of the ninth, but in the Physostomous Esox, the 

 the first pair of the post-plexus is the 18th, while in some Physoclists, 

 the first pair of the post-plexus is the fourth or fifth, a result which 

 makes the absurdity of such a method of reasoning still more pre- 

 posterous. The climax is reached when the theory of multiple 

 equivalency is resorted to in comparing the post-plexus oi Raia and 

 Esox. The thirty-third to the forty-sixth pair in Raia, fourteen in 

 all, would be equal to about three, or a little more, pairs in the 

 post-plexus of Esox, on the basis of the doctiine of multiples. This 

 is four less than the required number in Esox, besides which, the 

 first pair would be brought into the position of the Ainth instead of 

 the eighthteenth, its required place. Or if we took two pairs of 

 spinal nerves of Raia to equal one of Esox, we should only have 

 seven for the post-plexus of the latter or one less than the required 

 number, while the first would be brought into the position of the 

 seventeenth instead of the eighthteenth, its required position. 



The table also shows that the proplexus of Raia batis, according 

 to Swan, is nearly equal to the pro-and post-plexuses of Raia eglan- 

 teria taken together. It is also shown that the j)roplexus of 31u8- 

 telus canis is made up of not quite half as many nerve pairs as that 

 of Raia eglanieria, and it therefore has just a third as many as R. batis. 

 In that it is now known that each anterior somite gives ofi'just twice 

 as many muscular buds as there are somites, the buds corresponding 

 to as many rays, it is clear that the anterior fins of these three Elas- 

 mobranchs are not the exact homologues of each other, that is, the 

 pectoral of Raia is derived from more somites than that of Mustelus 

 and they can therefore not be equivalent in an exactly homological 

 sense. 



A still further inspection of the table Avill enable the reader to 

 make many other comparisons which will be equally striking. 

 Amongst the others, that of Phoccena is one of the most interesting. 

 It will be seen that the post-plexus is pushed backward. But this 

 may be partially accounted for, as only the first five pairs can be 

 fairly compared with those of other Mammals, this plexus in Phoccena 

 as a whole, having undergone reduction, the thirty -first to the forty-fifth 

 pairs forming what ought, perhaps, to be considered a caudal plexus. 



Throughout, it will be seen that from the Amphibia onward, the 

 number of nerve pairs entering into the formation of the plexuses, 

 have undergone as marked and abrupt a reduction in number, as the 



