52 PROF. T. H. HUXLEY ON CKRATODUS FORSTERI. [Jail. 4, 



cavity. In like manner the proximal prseaxial ray (propteryyium, R) 

 has ascended along the prseaxial face of the axial cartilage, until it 

 also is able to furnish a facet which completes the anterior part of 

 the cup for the condyle of the pectoral arch. 



In Squatina, similar modifications have occurred ; but the axial car- 

 tilage remains large, and the large prteaxial and postaxial cartilages 

 are directed respectively forwards and backwards, in accordance with 

 the form of the vastly expanded fin. 



In Raia, yet further expansion is obtained by the separation of 

 the axial and postaxial cartilages and the interpolation of postaxial 

 ravs between them. The proximal ends of these enter into the 

 articulation, as the great postaxial cartilage had already done. 



The interpretation of the skeleton of the pectoral fin of Chimcera 

 presents some difficulties. This skeleton consists (fig. 10) of: — (^)a 

 proximal cartilage (I), which articulates by an excavated surface with 

 the condyle of the pectoral arch ; (2) a flat, curved, elongated middle 

 cartilage (Mt), which is inclined backwards; (3) a small semi- 

 lunar distal cartilage (c), which fits onto the convex distal end of 

 the last. At the end of the convex posterior edge of the distal car- 

 tilage is a small cartilaginous raj% consisting of a long basal and a 

 short terminal segment. Similar rays, which gradually become longer, 

 follow this along the free convex edge of the distal cartilage and that 

 of the middle cartilage ; but the proximal end of the latter bears a 

 much stronger ray, with a wide base (R), which for the most part 

 unites with the ventral edge of the proximal cartilage (I), but is con- 

 nected with the middle cartilage (Mt) by its posterior produced 

 angle. 



There can, I think, be no doubt that the proximal cartilage (I) 

 in Chimeera answers to the proximal cartilage in Ceratodus. The 

 small proximal postaxial cartilages also exactly correspond ; and 

 the large proximal praeaxial rays no less closely answer to one 

 another. But if this be so, it follows that the whole skeleton of the 

 fin in Ceratodus is represented in that of Chimcera. The distal 

 cartilage (c) in Chimcera is the result of the coalescence of the bases 

 of a certain number of the postaxial rays, as is obvious on tracing the 

 scrips round. 



Hence it would appear that all that can represent the series of 

 median segments except the first is the middle cartilage (Mt). 

 It further seems probable that this middle cartilage in great part, 

 if not wholly, represents the second segment of the Ceratodus 

 limb. The postaxial edges, a b and b c, correspond closely ; but the 

 edge e f, long in Ceratodus, is reduced to nothing in Chimcera ; 

 while the edge c d, occupied exclusively by the third segment in 

 Ceratodus, is greatly elongated and bears all the prseaxial rays in 

 Chimcera. 



In order to change the skeleton of the pectoral fin of Ceratodus 

 into that of Chimcera, all that will be necessary, if this comparison 

 is correct, is that the third and following median segments of the 

 former shall be gradually reduced, either by abortion or coalescence 

 with the second, more and more postaxial fin-rays becoming attached 



