480 MAMMALIA. 



panic cavity is bounded ventrally by a tympanic bone, and the 

 quadrate is not present as a distinct element being probably 

 absorbed into the squamosal. There is 110 postfrontal bone and 

 only one temporal arcade (see p. 319). The vertebral centra 

 almost always carry epiphyses. There are almost always seven 

 cervical vertebrae. The coracoid is with one exception reduced, 

 and fused with the scapula, not reaching the sternum. Except 

 in whales, the pollex and hallux usually have two phalanges, the 

 other digits three. When the digits are reduced the order of 

 disappearance is almost always No. 1, No. 5, No. 2, No. 4, so that 

 if there is only one digit it is No. 3, if two Nos. 3 and 4. In the 

 brain the cerebellum has two lateral lobes as well as a median, 

 there are four optic lobes (corpora quadrigemina), and the cere- 

 bral hemispheres are connected by a broad commissure, the 

 corpus callosum. The portions of the body cavity containing 

 the lungs are always separated from the general body cavity by 

 a muscular septum, the midriff or diaphragm. There is a urino- 

 genital sinus, called urethra in the male and vestibule in the 

 female, which receives the opening of the bladder and of the 

 generative ducts ; it opens independently of and ventral to the 

 anus except in the Monotremata and a few other forms. 



The testes and ovaries have nearly always shifted backwards 

 in the body cavity from their primitive position near the kidney ; 

 in the case of the male the shifting is considerable and the testes 

 very generally lie in pouches of the body cavity called the scrotal 

 sacs, placed close to the root of the penis. A penis is always pre- 

 sent, consisting of two corpora cavernosa attached to the ischia 

 and of a corpus spongiosum which swells terminally to form the 

 glans penis. With very rare exceptions the penis is traversed 

 by the urethral canal. The female has a reduced homologue of 

 the penis, the clitoris, which is only rarely traversed by the 

 urethra. 



In addition to the peculiarities in the vascular system men- 

 tioned in the definition, viz. the four- chambered heart and the 

 presence of an aortic arch on the left side only, it ought to be 

 mentioned that there is no renal -portal circulation. 



The ovum is always small and provided with but little food 

 yolk. As might be expected the cleavage is total and the greater 

 part of the development (excepting in the Monotremata in which 

 the cleavage is partial) takes place in that portion of the oviduct, 



