ANATOMY OF VERTEBRATES. 131 



the shortest in the Dog-fish, and supports three or four rays ; the 

 posterior one is much longer, and supports the remainder of the 

 rays, fifteen or sixteen in number. To the end of this cartilage 

 likewise is attached, in the male Plagiostomes, the peculiar ac- 

 cessory generative organ or clasper. In the Torpedo the arch 

 sends forward two processes, and these are of greater length in 

 the extinct Cyclobates oligodactylies, XL. p. 225, pi. 5. In the 

 Chimeroids the short narrow processes which extend above 

 the place of articulation of the ventral fins simulate iliac bones : 

 the expanded portions which meet below represent the ischia ; 

 they are each of them perforated by a large round aperture, filled 

 by membrane. The cartilage, answering to the tibia, supports 

 the rays of the ventral fin and the clasper. 



§ 42. Pelvic arch and limb of Reptiles. — Passing from the 

 Protopterus, fig. 101, c, to the Proteus, ib. D, we find the 

 pair of cartilages answering to the piscine ' ischia' aided by 

 a second pair, 62, called ' ilia,' in supporting the diverging 

 appendage ; and this pair is attached to the riblets of the last 

 abdominal or ' sacral ' vertebra. The appendage or ' limb ' now 

 consists of definable segments, which are specialised through sub- 

 sequent developements : the first, as the ' femur ; ' the second — a 

 pair of shorter and smaller joints — respectively as ' tibia ' and 

 ' fibula ; ' these being followed, with the intervention of a cartila- 

 ginous ' tarsal ' mass, by a pair of many-jointed rays or ' digits.' 



A closer correspondence, however, with the piscine type was, 

 in some respects, retained by the extinct Ichthyosauri, fig. 105. 



Although the iliac element, 62, was joined with the ischial, 63, 

 in supporting the fin, such sustaining arch remained freely sus- 

 pended, as in the ( ventral fishes ' of Linnams. A second ha?ma- 

 pophysial arch, 64, was likewise present, in advance of the ischial 

 one, answering to the e pubis ; ' this afforded the extent of origin 

 required by the muscles of the better developed fin. The next step 

 in progress is exemplified by the single long and inferiorly flattened 

 and expanded bone, 65, answering to the ' femur,' and through 

 which, as on a pedicle, the fin could be more freely rotated, and 

 moved to and fro. To the end of the femur were ligamentously 

 articulated the two short flattened bones representing the tibia 

 and fibula ; followed by the series of multiarticulate digits, joined 

 together to form the common basis of the fin, which, like the 

 pectoral one, tapered to a point. 1 



With the increased length, and progressive differentiation of the 

 several segments of the fin, as in Plesiosaurus, fig. 45, the pubic 



1 No twist, real or imaginary, of humerus or femur obscures or is needed to 

 explain the homotypes in the pectoral and pelvic members, clx. 



