263 



CHAPTER LIX. 



CLASS MAMMALIA— continued. 



Orders Monotremata and Marsupialia. 



Subclass I. Prototheria. — This subclass, now represented by 

 only two genera, may be characterised as follows. The brain has 

 a large anterior commissure, and a very small corpus callosum ; 1 

 while the auditory ossicles are simple, and the stapes is rod-like 

 (columelliform). The coracoid is a distinct, although small, bone, 

 anchylosing in the adult to the scapula, and articulating with the 

 sternum ; while there is a separate precoracoid (epicoracoid), which 

 does not articulate with the scapula, and also a large T-shaped inter- 

 clavicle ; the form and relations of these bones being very like those 

 of the corresponding parts of the skeleton in the Anomodont Rep- 

 tiles. The pelvis has epipubic bones, and the ilia are inclined to 

 the sacral axis after the Batrachian fashion ; thus resembling those 

 of the Pariasaurian Anomodonts. The urinogenital and excretory 

 organs open into a common outlet, or cloaca ; and the former are 

 very similar to those of the Sauropsida ; the mammary glands are 

 unprovided with nipples, and the reproduction is oviparous ; the 

 eggs being meroblastic like those of Birds. 



It may here be observed that the small bone in the pectoral girdle of 

 the Monotremes placed in advance of the coracoid, which is usually 

 termed the epicoracoid, appears to correspond with the precoracoid of 

 the Anomodont Reptiles 2 (fig. 978 bis, p. 1054), although it does not ex- 

 tend upwards to articulate with the acromial process of the scapula, as 

 the Anomodont precoracoid articulates with the process of the scapula 

 identified by Sir R. Owen with the acromion. 3 Further, the scapula of 

 the Monotremes differs from that of all other Mammals, and resembles 



1 The structure connecting the two hemispheres of the brain. 



2 This appears to be the view taken by Professor Cope. 



3 This is the original view. In describing the scapula of Plaiypodosaurus, 

 Sir R. Owen confused the process situated above a in fig. 978 bis with that 

 marked a, and termed the former the acromion. 



