ANATOMY OF VERTEBRATES. 



179 



112 



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

 The peculiarity of the bones c and d in Crocodilia, is their unusual 

 length, showing a constricted shaft between the expanded ends, 

 thus simulating metacarpals in shape, for which, when found as 

 detached fossils, they have been mistaken. 



The digits in the 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 phalanges ; in Test, tabulata the fifth digit has but one 

 phalanx. 1 



§ 41. Pelvic arch and limb of Fishes. — Some cold-blooded verte- 

 brates, e.g. Muraenoids and Ophidians, have neither fore nor hind 

 limbs. In a few, e. g. Anguillidce, Gymnoti, 

 Xiphias, Siren, the scapular arch and limbs 

 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 hEemapophysial 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. 112, 68, are directly supported by the bones, 

 68, which, uniting together, near the point of 

 attachment of the fins without an intervening 

 bone, resemble their homotypes, the coracoids ; 

 by virtue of which ' serial homology,' we 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 flesh, 

 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 Malacopterans all the ventral 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 appears in the developement of the embryo fish, as 



1 xliv. p. 205, No. 1079 : see also pp. 206, 207, Nos. 1079, 1083, 1087, 109], 

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



N 2 



Pelvic arch and limbs, 

 Trout (Salmo) 



