MAMMALS. 



Abranchiate Vertebrates with a brain whose cerebral hemispneres are more or less 

 connected (and in nearly inverse ratio) by an anterior commissure, and a superior 

 transverse commissure (corpus callosum) ; the latter more or less roofing in the lateral 

 ventricles : lungs and heart in the thorax, separated from the abdominal viscera 

 by a muscular diaphragm : aorta single and reflected over the left bronchus : blood 

 with red non-nucleated blood-corpuscles ; undergoing a complete circulation, being 

 entirely received and transmitted by the right half of the quadrilocular heart to the 

 lungs for aeration, (and warmed,) and afterwards returned by the other half through 

 the system. Skull with two condyles, chiefly developed on the exoccipital elements, 

 (one on each side of the foramen magnum): with the malleus and incus superadded as 

 specialized auditory ossicles : and the lower jaw (composed of a pair of simple rami) 

 articulated directly by convex condyles with the squamosal bones. Dermal append- 

 ages developed as hairs. Viviparous : foetus developed from a minute egg : young 

 nourished after birth by a fluid (milk) secreted in peculiar glands (mammary) by 

 the mother. 



SUB-CLASSES. 



I. Brain with superior transverse commissure composed of a body as well as 

 psalterial fibres ; and with a well developed septum. Sternum with 

 no element in front of the manubrium or presternum. Coracoid not con- 

 nected with the sternum, but early anchylosed with and developed as a 

 simple process of the scapula. Oviducts debouching into a double or single 

 vagina, (and not into a common cloacal chamber). Testes variable in po- 

 sition, but the vasa deferentia open directly or indirectly into a distinct 

 and complete urethra, (and not into a cloacal cavity). Ureters discharge 

 directly into the bladder the renal secretion, which thence passes into the 

 urethra. Mammary glands with well developed nipples. 



A. Brain with the cerebral hemispheres connected by a more or less well- 

 developed corpus callosura and a reduced anterior commissure. Vagina 

 a single tube, but sometimes (vith a partial septum. Young retained 

 within the womb till of considerable size and nearly perfect development, 

 and deriving its nourishment from the mother through the intervention 

 of a "placenta" (developed from the allantois) till birth. Scrotum 

 never in front of penis. 



MONODELPHIA. (I.) 



B. Brain with the cerebral hemispheres chiefly connected by a well-de- 

 veloped anterior commissure, the corpus callosum being rudimentary. 



