314 THE PELVIC GIRDLE. [CHAP. 



osseous pieces always coalesce into a single bone, called the 

 os innominatum. 



This is further completed by the addition of epiphyses ; 

 one for the upper extremity of the ilium (corresponding to 

 the supra-scapular epiphysis of the shoulder), and others for 

 the most prominent parts of the lower or free borders of the 

 pubis and ischium (symphysis pubis and tuber ischii). 



In most Mammalia (perhaps with the exception of the 

 Monotremata, Lemuroidea and Chiroptera) a fourth pelvic 

 bone is found, situated between the ilium, pubis, and ischium. 

 This, the so-called os acetabuli^- either persists as a separate 

 ossification (it is especially large in Talpa, Sorex, Viverra, 

 and Lepus) or it fuses with one of the three other pelvic 

 bones. Its morphological meaning is as yet unknown, but 

 it can scarcely be considered as an epiphysis. 



The three principal bones of the pelvis nearly always 

 enter into the formation of the acetabulum, but in a few 

 genera, as Lepus, \\\e pubis is excluded. 2 



There is never any secondary osseous bar in the pelvic 

 girdle corresponding to the clavicle of the upper extremity. 



The ilium of Mammals is essentially an elongated, three- 

 sided, or prismatic bone, though the relative size and posi- 

 tion of the various surfaces and angles may differ greatly 

 in different species. In the most characteristic form, one 

 of the surfaces is internal, or directed towards the ver- 

 tebral column, articulating by a flat irregular surface with the 

 lateral " pleurapophysial " ossifications of the sacral vertebrae. 



1 See W. Krause, " Ueber den Pfannenknochen," Centralblatt f. 

 d. medicinisch. \Vissenschaft. 1876, No. 46 ; W. Leche, " Zur 

 Anatomic der Beckenregion bei Insectivora." K. Svenska Vetensk. ; 

 Akad.'s Handlingar, xx. 1883 ; and Bronn's " Clas en und Ordnungen 

 des Thierreichs, Mammalia." 



2 Gegenbaur, " Ueber den Ausschluss der Schambeines vom Hueft- 

 gelenk," Morph. Jahrb. 1876. 



