13 



abdominal sternum. The pubic and ischial arches are gene- 

 rally complete, and united together by bony confluence on the 

 sternal aspect, so that the interspace of the two pelvic arches 

 is converted into two holes, called ' foramina obturatoria.' 



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 tympa- 

 num ; the membrana tympani is generally concave externally ; 

 the meatus auditorius externus often commences with a com- 

 plicated 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 communicate 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 

 'zygomatic' bar continued backward 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 osteo- 

 logical characters present. 



The cancellous texture of mammalian Ibone is of a finer 

 arid more delicate structure than in Keptiles, and forms a 



