324 THE PELVIC GIRDLE. [CHAP. 



the same species, and often on the two sides of the same 

 animal. 



In the Dolphins, they are generally smaller, and more 

 simple than in the Whalebone Whales, and usually quite 

 straight, though sometimes arched, or presenting a sigmoid 

 curve. In Platanista alone no traces of pelvic bones have 

 yet been found. 



Order EDENTATA. In the Sloths, the pelvis is very short 

 and wide, with tolerably broad flattened ilia, and slender 

 pubes and ischia, inclosing a large oval thyroid foramen, 

 the inferior boundary of which, and the extremely narrow 

 ossified symphysis, are formed by the pubis alone. The 

 spine of the ischium is produced backwards to unite with 

 the transverse processes of some of the pseudo-sacral ver- 

 tebrae, inclosing a sacro-sciatic foramen. The sacro-iliac 

 articulation is commonly ankylosed. 



In all the other Edentates the pelvis is more or less elon- 

 gated, the ilia trihedral, the ischia largely developed, the 

 pubes slender, the symphysis exceedingly short, but usually 

 ossified, and the thyroid foramen large. In all, except Oryc 

 teropus, the ischia unite with the vertebral column. This 

 union is carried to its greatest extent in the Armadillos, in 

 which animals the broad transverse processes of as many as 

 five pseudo-sacral vertebrae may coalesce with each other and 

 with the side of the ischium, converting the pelvis into a long 

 bony tube, the more so as the ilia are also firmly and exten- 

 sively united with the true sacrum. There is usually, especially 

 in Orycteropus, a strongly developed " pectineal " tubercle. 



Order MARSUPIALIA. In the American Opossums 

 (Didelplivs], the ilium is a very simple, straight, three-sided 

 rod, thicker at its upper end than at its acetabular end, 

 each side being nearly equal in extent, hollowed, and sharply 

 defined by prominent straight borders. 



