PELVIC GIRDLE. 499 



a single ossification in its posterior part which forms the ischium ; while 

 the anterior part, which may be considered as representing the pubis, 

 usually remains cartilaginous; though Huxley (No. 475) states that it has 

 a sepai'ate centre of ossification in Salamander, which however does not 

 appear to be always present (Bunge). There is a small obturator foramen 

 between the ischium and pubis, which gives passage to the obturator nerve. 

 It is formed by the part of the tissue where the nerve is placed not be- 

 coming converted into cartilage. 



There is a peculiar cartilage in the ventral median line in front of the 

 pubis, which is developed independently of and much later than the true 

 parts of the pelvic girdle. It may be called the prpepubic cartilage. 



Lacertilia. In Lacertilia the pelvic girdle is formed as a somewhat 

 triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two 

 ventral (pubic and ischiad) processes. The acetabulum is placed on the 

 outer side at the junction of the three processes, each of which may be 

 considered to have a share in forming it. The distal ends of the pubis 

 and ischium are close together when first formed, but subsequently 

 separate. Each of them unites at a late stage with the corresponding 

 process of the opposite side in a ventral sympbysis. A centre of ossifica- 

 tion appeai-s in each of the three processes of the primitive cartilage. 



In Birds the parts of the pelvic girdle no longer develop as a continu- 

 ous cartilage (Bunge). Either the pubis may be distinct, or, as in the Duck, 

 all the elements. The ilium early exhibits a short anterior process, but 

 the pubis and ischium are at first placed with their long axes at right 

 angles to that of the ilium, but gradually become rotated so as to lie paral- 

 lel with it, their distal ends pointing backwards, and not uniting ventrally 

 excepting in one or two Struthious forms. 



Mammalia. In Mammalia the pelvic girdle is formed in cartilage 

 as in the lower forms, but in Man at any rate the pubic part of the carti- 

 lage is formed independently of the remainder (Rosenberg). There are 

 the usual three centres of ossification, which unite eventually into a single 

 bone the innominate bone. The pubis and ischium of each side unite with 

 each other ventrally, so as completely to enclose the obturator foramen. 



Huxley holds that the so-called marsupial bones of Monotrernes and 

 Marsupials, which as shewn by Gegenbaur (No. 474) are preformed in carti- 

 lage, are homologous with the praepubis of the Uroclela ; but considering 

 the great gap between the Uroclela and Mammalia this homology can only 

 be regarded as tentative. He further holds that the anterior prolongations 

 of the cartilaginous ventral ends of the pubis of Crocodilia are also struc- 

 tures of the same nature. 



BIBLIOGRAPHY. 



(473) A. Bunge. T'nler^irJi. z. Entirick. d. Beckengiirtel* d. Amphilien, Reptilien 

 n. Vi'iqel. Inaug. Diss. Dorpat, 1880. 



(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne 

 d. Hiiftgelenkes." Morph. Jahrlntch, Vol. n. 1876. 



(475) Th. H. Huxley. " The characters of the Pelvis in Mammalia, etc." Proc. 

 of Boy. Soc., Vol. xxvm. 1879. 



(476) A. Sabatier. Comparaison des ceintures et des membrcs anUrieun ft 

 posterieurs daiis la Serie d. Vertebres. Montpellier, 1880. 



322 



