356 PROCEEDINGS OF THE ACADEMY OF [1887. 



themselves of the theory of the translocation of the limbs, left with 

 nothing to rely upon. Even supposing that the phylogenetic history 

 is inaccurately repeated in the ontogeny of the forms here used in 

 illustration, how will we explain on such a supposition, the persist- 

 ent posterior origin of the pelvic limbs in Physostomes from folds 

 separated by an interval of as many as 20 somites from the pectorals, 

 while in Physoclists there is either a very small interval or almost 

 none ; and why is it that the nerve supply for the limb should be 

 derived so constantly from the pairs underlying the point of origin 

 of the limb-fold ? We saw, moreover, that when translocation did 

 occur during ontogeny, that the distal portion of the nerve supply 

 was carried along with the limb, while the point of the origin of the 

 nerve supply remained unchanged. The other cases of translocation, 

 which are well made out, the Cetacea and Lyomeri are not fairly to 

 be contrasted with that of the Physoclisti, because, in the first in- 

 stance the hind limbs have become totally incapable of free or 

 independent movement, while in the case of the Lyomeri, the fore-limbs 

 have become detached from the cranium, and limb and pectoral arch 

 so undeveloped as to present a condition which is attained by many 

 fresh-water forms before they leave the egg. The Cetacea, Lyomeri 

 and Ichthyosauria, are the only vertebrates in which the pelvic or 

 pectoral girdle has suffered displacement, detachment or reduction, 

 except the Physoclisti, and, while it must be admitted that our in- 

 formation as to how this occurred, is for the most part only inferen- 

 tial and based upon comparative studies of the morphology of allied 

 forms, as far as regards the first three, the actual obserVStion of how 

 such a process has occurred in the last named group should make us 

 all the more ready to expect equally remarkable revelations regard- 

 ing the others. And of fossil groups besides the Ichthyosauria, it 

 is not impossible or improbable that such as the Enaliosauria and 

 others, might afford profitable and suggestive contrasts. Tliese, 

 however, we can have little hope of studying from the stand-point 

 of the embryologist, owing to the paucity of material. 



The embryological evidence has now been stated, together with 

 such a discussion of the extent to which translocation of the limbs, 

 during their early stages, has had any bearing upon the questions at 

 issue, so that we nuiy at once turn to the consideration of the mor- 

 phology of adult forms in this connection. 



2. . Anatomical evidoice, considering esjyecially the relations of the 

 spinal nerves to the limbs. 



