BY WILLIAM A. HAS WELL, M.A., B.S.C. 5 



nected with tlie primitive form o£ the appendicular skeleton ; 

 and, as we compare the girdle with a branchial arch, so we may 

 compare the median ray and its secondary investment of rays 

 with the skeleton of the free appendage." 



" All the varied forms which the skeleton of the free appendages 

 exhibits may be derived from a ground-form which persists in a 

 few cases only, and which represents the first, and consequently 

 the lowest, stages of the skeleton of the fin — the Archipterygium. 

 This is made up of a stem which consists of jointed pieces of 

 cartilage, which is articulated to the shoulder-girdle, and is beset 

 on either side with rays which are likewise jointed. In addition 

 to the rays on the stem there are others which are directly 

 attached to the limb-girdle." 



" Ceratodus has a fin-skeleton of this form ; in it there is a 

 stem beset with two rows of rays. But there are no rays on the 

 shoulder girdle. This biserial investment of rays on the stem of 

 the fin may also undergo various kinds of modifications. Among 

 the Dipnoi, Frotopterus retains the medial row of rays only 

 which have the form of fine rods of cartilage ; in the SeJacldi, 

 on the other hand, the lateral rays are considerably developed." 



Thus both Gregenbaur and Huxley regard the fin of Ceratodus 

 as representing or nearly representing a primitive type from 

 which the limbs of the Ganoids, the Holocepliali, and the Selachii 

 have been derived. E-easons have already been adduced by 

 Balfour,* Thackerand others, for dissenting from this conclusion, 

 and these, together with the facts which I am about to bring 

 forward, seem to me to place it beyond a doubt that the limb of 

 Ceratodus, so far from representing a primitive and generalised 

 type, is, as indeed we should expect from various other points in 

 the organisation of the animal, in reality highly specialised, and 



* Comparative Embryology, Vol. ii., p. 506 ; " On the Development of the 

 Paired Fins of the Elasmobrauchii," etc., P.Z.S., 1881, p. 656. 



