4 PEOFESSOE OWEN ON THE CHAEACTERS, ETC. 



the interposition of a distinct articulation : there are no gristly or 

 bony abdominal ribs or abdominal sternum. The pubic and ischial 

 arches are generally complete, and united together by bony con- 

 fluence on the sternal aspect, so that the interspace of the two 

 pelvic arches is converted into two holes, c2iW.edi foramina ohtura- 

 toria or thyroidea. The sclerotic coat of the eye is a fibrous 

 membrane, and never contains bony plates. In the quantity of 

 aqueous humour and the convexity of the lens Mammals are 

 generally intermediate between Birds and Fishes. The organ of 

 hearing is characterized by the full development of the cochlea 

 with a lamina spiralis : there are three distinct ossicles in the 

 tympanum ; the membrana tympani is generally concave exter- 

 nally ; the meatus auditorius externus often commences with 

 a complicated external ear, having a distinct cartilaginous basis. 

 The external apertures of the organ of smell are provided with 

 moveable cartilages and muscles, and the extent of the internal 

 organ is increased by accessory cavities or sinuses which commu- 

 nicate with the passages including the turbinated bones. 



There are few characters of the osseous system common, and at 

 the same time peculiar, to the class Mammalia. The following 

 may be cited : — 



1. Each half or ramus of the mandible consists of one bony 

 piece developed from a single centre: the condyle is convex or 

 flat, never concave. This has proved a valuable character in the 

 determination of fossils. 



2. The second or distal bone, called " squamosal," in the bar con- 

 tinued backwards from the maxillary arch, is not only expanded, 

 but is applied to the side-wall of the cranium, and developes the 

 articular surface for the mandible, which surface is either concave 

 or flat*. 



3. The presphenoid is developed from a centre distinct from 

 that of the basisphenoid. 



In no other class of vertebrate animals are these osteological 

 characters present. 



The cancellous texture of mammalian bone is of a finer and 

 more delicate structure than in Keptiles, and forms a closer net- 

 work than in Birds. The microscopic radiating cells are relatively 

 smaller and approach more nearly to the spheroid form ; but both 

 these histological characters are liable to mislead, if unsupported 

 by more obvious and constant ones, in the interpretation of a 

 fossil. 



* The Wombat is, perhaps, the sole exception to this rule. 



