ANATOMf OF VERTEBRATES. 



179 



112 



to which is added a pisiibmie, fig. 109, e, in the usual position. 

 The peculiarity of the hones c and d in CrocodiUa, is their unusual 

 length, sliowing a constricted sliaft ))etwcen tlie expanded ends, 

 tlms simulating metacarpals in shape, for which, wlicn found as 

 detached fossils, they have been mistahen. 



Tlie digits in tlie class Reptilia are generally characterised by 

 a progressive increase in the number of their joints, from the first 

 or innermost to the third or fourth ; the Chelonia being the chief 

 exceptions. In Testudo, fig. 108, each digit has one metacarpal 

 and two pjhalanges ; in Test, tdhidata the fifth digit lias but one 

 phalanx.' 



§41. Peloic arch and liinh of Fis/ie.i. — Some cold-blooded verte- 

 brates, e. g. Muraanoids and Opliidians, have neither fore nor hind 

 limbs. In a few, e. g. Anr/iuUidfe, Gyiintoti, 

 Xijdiias, Sirrn, the scapidar arcli and limlis 

 are present, but not the pelvic arch and limbs. 

 In most fishes the latter exist, but less deve- 

 loped than the pectoral ones, and less fixed in 

 position. 



Only the hffimajiophysial portion of the 

 pelvic arch is developed in connection with 

 the diverging appendages, termed in Fishes the 

 ' ventral fins.' Their rays in osseous fishes, 

 fig. 1 12, 68, are directly supported by the bones, 

 r,s, which, uniting together, near the point of 

 attachment of the fins without an intervening 

 bone, resemble their homotypcs, the coracoids ; 

 by virtue of whicli ' serial homology,' Ave infer 

 their special one with the ischia of higher 

 animals. Each bone is a subtriangular plate, supporting the 

 fin by its expanded end; and, either suspended in the fiesh, 

 as in the Salmon and Sturgeon, fig. 29, v, or attached by the 

 narrower end to the coracoid, 52, as in the Cod, fig. 36, 63. The 

 representatives of tarsal, tibial, and femoral bones, are wanting in 

 all Fishes. In Acanthopterans one or more of the anterior rays 

 of the ventral fin may be hard unjointed spines, as in the other 

 fins ; in Malaeopterans all the A-entral rays are soft, multiarti- 

 culate, and bifurcate. 



In no fish is this incomplete pelvic arch directly attached to the 

 vertebral column. If we may judge from the position in which 

 the ventral fin aj.ipears in the developement of the embryo fish, as 



' XLiv. p. 205, No. 1079 ; see also pp. 206, 207, Nos. 1079, 1083, 1087 

 1093 for further details of the bony structure of the fore foot in Chelonia. 



I09I 



