Prof: Owen on the Class Mammalia. 9 
pore ae are ena ntti, a sometimes, as in the a of 
certain Ruminants, they are concave behind and convex in front: 
such a vertebra, however, may be distinguished from a vertebra 
of a Reptile, with a similar ball-and socket structure of the artic: 
ular surfaces, even when found in a fossil state, and when the 
test of the articulating medium cannot be applied, by the com- 
plete anchylosis or confluence of the annular with the central 
part or a and by the large relative size of the canal for the 
spinal chord. The cervical ‘verte re, with one or two excep 
teens are seven in number, neither more nor less: the Mono- 
tremes, which are the instances commonly opposed to other gen- 
eralizations, form no exception to this rule. aa lumbar vertebrae 
are pikes constant and usually more numerous than im other 
classes of vertebrate animals, The atlas is articulated by con- 
cave ieucaiar processes to two convex condyles, which are 
developed from the ex-occipital elements of the last cranial ver- 
tebra. The typanic element of the temporal bone is r —— 
in function to the service of the organ of hearing, and n 
enters into the articulation of the lower jaw. The elaetory 
nerves escape from the cranial cavity through numerous foram 
of a cribriform _ The optic foramina are always dase 
from one another 
he scapula is generally an expanded plate of bone; the 
coracoid, with two (monotrematous) exceptions, appears as a 
small process of the scapula. The sternum consists of a narrow , 
i aes simple series of bones : the sternal portions of the - 
The pubic and ischial arches are hae eles and united 
together by bony confluence on the sternal aspect, so that the 
interspace of the two pelvic arches 3s converted into two holes, 
called foramina obturatorfa or thyroidea. The sclerotic eoat of the 
eye is a fibrous oS and never contains bony plates. In 
the quantity of aque s humor and the convexity of the lens 
Mammals are ge perally intermediate between Birds and Fishes. 
The organ of hearing is characterized by the full development 
of the olan with a lamina spiralis: there are three distinet 
ossicles in the tympanum; the membrana tympani is generally 
_ concave externally; the meatus auditorius externus often com- 
mences with a complicated external ear, degrees: a distinet cartil+ 
aginous basis. The external apertures of the organ of smell 
SECOND SERIES, VOL. XXV, NO. 73.——JAN., 1858, 
2 
