1887.] NATURAL SCIENCES OF PHILADELPHIA. 357 



In order to make coruparisou easy between the various adult types, 

 I have tabulated the pairs of nerves which enter into the composi- 

 tion of the brachial and lumbo-sacral j)lexuses, or, as we may name 

 then) for brevity's sake j^'^'^P^^^^^-^ ^^^ postplexus, in such a way as 

 to show their inexact homology and homonymy at a glance. The 

 pairs of nerves belonging to the proplexus are indicated by the sign 

 X, while those belonging to the series of the postplexus are indicated 

 by the sign o; these signs are also placed in the successive columns 

 from left to right in such an order as to indicate at once the serial 

 number of the nerves they stand for, as reckoned from the occipital 

 foramen towards the end of the tail. 



The relations of the proplexus and postplexus may be graphically 

 represented by such a table and it is evident from a simple inspec- 

 tion and comparison of the limb-innervating series of spinal nerves 

 indicated by the arbitary signs, that there is clearly a lack of ho- 

 monymy of the paired limbs, if the determination of such homonymy 

 depends upon the origin of the paired limbs from somites, which are 

 distant the same number of segments from the occiput or posterior 

 extremity of the head. 



The thirty-two pairs belonging to the proplexus of Raia eglanteria 

 cannot by any possible supposition be made equivalent to the third 

 and fourth nerves entering into the proplexus of Rana; nor can it 

 be shown that the proplexus of Raia is exactly homologous with 

 that of Esox or Cottus. Even supposing that every two or three 

 pairs of Raia represent morphologically, but a single pair in Rana 

 and Esox, the difficulty is not disposed of, because, while it is possi- 

 ble to suppose that one-half of a given number of somites in a Sela- 

 chian, represent a morpliologically equivalent number in Teleosts or 

 Amphibia, there is no valid ground anywhere discoverable in mor- 

 phology, for supposing that the ratio of the morphological equiv- 

 alency, when the values ( 'f the somites of the two types are contrasted, 

 is greater than this. This contrast, however, does not avail, for 

 even upon such a supposition, one-half of the number of nerve pairs 

 in the proplexus of Raia is sixteen, or eleven more than in Esox. 



The utter absurdity of this last way of bringing the proplexuses 

 of Raia and Esox into homonymical agreement is brought out, if 

 we make another contrast. In fact, it is j)ossible to show that, if we 

 push the theory of multiples too far that we cannot account for the 

 relations of the post-plexuses of a Physostome and a Physoclist. 

 For, suppose the thirty -two pairs of the proplexus of Raia eglanteria 



