LIMBS. 99 



border of the pubes, right and left of the middle line, and extend 

 forwards in a straight or oblique direction. At present no satis- 

 factory morphological explanation has been given of them, and 

 we can only compare them with the epipubic cartilage of Dipnoi 

 and Amphibia, which has the same relations to the pyramidalis 

 muscle (comp. p. 117). Fibrous rudiments of them are to be seen 

 among Dogs. 



Before leaving the pelvic arch it may be pointed out that, 

 like the pectoral, it is not restricted to any particular body- 

 segment, but that both present much variety as to position, 

 phylogenetically as well as outogenetically. 



FREE LIMBS. 



Fishes and Dipnoi. In the Dipnoi, taking Ceratodus more 

 particularly into consideration, both pectoral and pelvic fins are 

 supported by a cartilaginous axis, made up of a great number of 

 small segments which are movable upon one another. 



Numerous small jointed cartilaginous rods or radii are disposed 

 serially along the dorsal and ventral sides of this axis, and these 

 gradually decrease in size towards its distal end. Towards the 

 periphery of the fin the place of these cartilages is taken by fine 

 horny rays, which are covered by fibrous tissue as well as by the 

 skin; thus a broad paddle-like fin is formed. Both pectoral and 

 pelvic fins have a similar form and structure, and in their natural 

 position a lateral (external) and a medial (internal) surface can 

 be distinguished. The dorsal radii are much more numerous 

 than the ventral, which have undergone reduction. Thus the 

 biserial type of fin is already modified in Ceratodus, and this 

 modification is carried still further in Elasrnobranchs (cornp. 

 Fig. 86), until eventually on]y a single series of radii (Fig. 69, 2>a') 

 remains. This series corresponds to the dorsal row of Ceratodus, 

 but in consequence of its position in the adult fin, where it divides 

 a dorsal from a ventral surface, it is spoken of as lateral. 1 

 The radii are much jointed, the segments being arranged in a 

 mosaic, and closely bound together by fibrous tissue. They are 

 covered over by the shagreen-like skin, and are continued out- 

 wards towards the periphery of the fin by a large number of horny 

 rays (Fig. 69, Fty, so that the size of the fin is thus considerably in- 

 creased. Three larger cartilaginous basal elements lie proximally 

 to the small radii, and are spoken of as pro-, meso-, and meta- 

 pterygium respectively (Fig. 69, Pr, Ms, Mf). They are connected 

 with the pectoral arch (SIT), and the metapterygium (Mt] together 

 with the distal elements lying along the same axis (a, 1} represents 



1 Fig. 54 shows how much reduction the skeleton of the fins has undergone in 

 Frotopterus, the lateral rays having almost entirely disappeared. The whole ex- 

 tremity consists simply of a long segmented cartilaginous iilanient, which no longer 

 serves as an organ of locomotion. 



H 2 



