FINS 155 



in the Primates, the ischia no longer meet below, and the broaden- 

 ing of the ilia seen in the higher forms of the last named Order 

 culminates in Man. The greatest amount of variety in the form 

 of the pelvis in any one order with typical appendages is seen in 

 Irisectivores, in some of which (e.g. Mole, Shrew), as well as in 

 most Bats, there is no symphysis pubis, so that the relatively small 

 pelvic cavity is not enclosed ventrally by bone. The obturator 

 foramen is always surrounded by bone. 



In the Cetacea, in which hind limbs are wanting, paired 

 vestiges of the ischiopubic region of the pelvis are present : they 

 are unconnected with one another and with the vertebral column. 

 In the Sirenia.a paired bony rod (Manatus) or plate (Halicore) repre- 

 sents the last vestige of an ilium, in which an ischium is included 

 in the latter genus. 



In Monotremes and Marsupials of both sexes, two strong so- 

 called "marsupial bones" (Fig. 116) arise from the anterior 

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

 forward in a stright or oblique direction embedded in the body- 

 walls, serving for the attachment of muscles. They form an 

 integral part of the pelvis, and in the embryo are seen to be in 

 direct connection with its cartilaginous symphysis (Fig. 116, B) ; 

 but later on definite articulations are formed between them and 

 the pubes (A). It is not improbable that these structures are 

 the homologues of the epipubis of lower Vertebrates, which has 

 been retained in non-placental Mammals in order to serve as a 

 support for the abdominal walls in connection with the marsupial 

 pouch. 



PAIRED FINS OF FISHES. 

 Fishes. 



The development of the extremities has already been alluded 

 to (p. 137). The pelvic fin usually retains a simpler and more 

 primitive form than the pectoral fin. 



Elasniobmnchs. The cartilaginous skeleton of the fins is the 

 most richly segmented in these Fishes. There are usually two main 

 elements (basalia)in the pelvic fin which articulate with the arch and 

 with which a variable number of segmented rays are connected, the 

 latter passing towards the periphery of the fin (Fig. 117). Both 

 the larger, posterior lasipterygium or rnetaptcryyiuiii, and the 

 smaller, inconstant propterygium must be looked upon as originat- 

 ing phylogenetically by a fusion of the proximal ends of the primary 

 cartilaginous rays of the fin ; and the form and relations of these 

 main elements vary according to the degree in which such a fusion 



