THE SKELETON IN MAMMALIA. PELVIC GIRDLE. 513 



Cetacea consists of two halves, usually united with one another 

 at the symphysis in the ID id-ventral line, and connected near 

 their upper ends, with the sacral vertebrae. Each half forms 

 one of the innominate bones, and includes at least three separate 

 elements, a dorsal bone, the ilium, and two ventral bones, the 

 ischium and pubis. Very often a fourth pelvic element, the 

 acetabular or cotyloid bone, occurs. 



In the MONOTREMATA the pelvis is short and broad, and the 

 pubes and ischia meet in a long symphysis. The acetabulum 

 is perforated in Echidna as in birds, but not in Ornithorhynchus. 

 A pair of elongated slender bones project forwards from the 

 edge of the pubes near the symphysis ; these are sesamoid bones 

 formed by ossifications in the tendons of the external oblique 

 abdominal muscles, and are generally called marsupial bones. 



In the MARSUPIALIA the ilia are generally very simple, 

 straight, and narrow, while the pubes and ischia are well de- 

 veloped and meet in a long symphysis. Marsupial bones are 

 nearly always prominent, but are not developed in Thylacinus 

 or Notoryctes. The ischium often has a well-marked tuberosity 

 and in Kangaroos the pubis bears a prominent pectineal 

 process on its anterior border close to the acetabulum. The 

 pelvis in Notoryctes differs much from that in all other Marsu- 

 pials, the ilium and ischium being ankylosed with six vertebrae 

 in a manner comparable to that of many Edentates. 



In the EDENTATA the pelvis is generally well developed, but 

 the symphysis is very short. In the Sloths the pelvis is rather 

 weak and slender, the obturator foramina are very large and 

 the ischia do not meet in a symphysis. In the Megatheriidae 

 the pelvis is exceedingly wide and massive, and is firmly 

 ankylosed with a number of vertebrae. In the Armadillos, 

 Glyptodonts, Anteaters, and Pangolins it is much developed 

 and firmly united to the vertebral column by both the ilia and 

 the ischia. In Orycteropus however the ischium does not 

 become united to the vertebral column, and the pubis generally 

 has a strongly developed pectineal process. 



a. 33 



