346 PROCEEDINGS OF THE ACADEMY OF [1887. 



be regarded as forms ancestrally or pliyletically related in a direct 

 line to the limb-bearing vertebrates. It will probably be best to 

 regard all of these, including the Lampreys and Hags, as extreme 

 retrograde modifications or as adumbrations of something higher in 

 the Chordate series, and indeterminate in every respect to their ex- 

 act position with reference to the great limb-bearing phylum. The 

 Elasmobranchii, therefore, alone remain as a point of departure. 



It is not possible to develop a rational interpretation of the inex- 

 act homology of the paired limbs of the various types, in which it 

 may be suspected to exist, unless we start with the Elasmohranchs^ 

 because, in the latter only, do we find the limbs in a condition which 

 there is every reason to regard as the most primitive. The princi- 

 pal mark of this primitiveness is, as before stated, the continuity, at 

 a certain stage, of the limb-rudiments, the elements of which are, 

 moreover, metamerically repeated, that is, they recur as out-growths 

 from each and every successive myotome of the series underlying 

 the paired integumentary folds, from portions of which, together 

 Avith a greater or lesser number of the underlying muscular limb- 

 buds and other mesoblast, the permanent limbs are finally diifer- 

 entiated. It is probable, therefore, that the lateral limb-folds of this 

 primitive type may be regarded as typifying almost completely the 

 ideal form from which all of the various types of jDaired limbs have 

 been evolved, as seen in the various groups. That such a general- 

 ized (not archetypal) ancestral form may be assumed to be repre- 

 sented by the most generalized Elasmobranchs, (Rays and Torpedos) 

 will, I believe, be fully justified by the evidence, which remains to 

 be offered in what is to follow. 



While the method by which limbs are developed in Sharks and 

 True Fishes, must be admitted by anatomists to be primitive, it 

 must not be forgotten that long limbs of functional value, such as 

 are possessed by Land Mammals, Birds and Reptiles, would be next 

 to useless and an actual impediment in the struggle for existence, if 

 appended to a fish. That this is true, is proved by the fin or pad- 

 dle-like limbs of Plesiosaurs, Ichthyosaurs, "Cetacea, Sirenia and 

 Pinnipeds, in all of which there has occurred a gradual abbreviation, 

 modification and even change of the position of the limbs, in order 

 to economize , the exertion of effort in' a dense medium water in 

 which short, fin-like limbs only would be preserved by mechanical 

 selection. That is, the limb which presented the greatest mechanical 

 advantages would be the one preserved, while its adaptation modi- 



