MAMMALS. 195 



GENEEAL REMARKS ON ALL THE MAMMALIAN ORDERS ABOVE 



THE MONOTREMATA. 



HAVING described the structures of the Shoulder and lower half of the thorax showing the 

 primordial distinctness of these regions, and their correlation in the Ovipara, and in the almost 

 Oviparous Monotrematous Mammals, I shall now show the advanced morphological condition of 

 the highest Vertebrate types the frequent lessening of the masses of skeletal substance, combined 

 with a higher metamorphic grade. This had already begun to show itself in the Birds ; but is 

 much more perfect in the Mammalia. 1 In the Marsupials, and in the rest of the Mammalia, the 

 Shoulder-plate is aborted below ; the scapula has a more or less developed supra-scapular 

 margin, either remaining soft, or being feebly ossified by one or more endosteal patches, or by 

 the creeping upwards of such deposit from within the main bone. The prse-scapular margin is 

 largely developed as a rule ; and sometimes it is developed as a separate cartilage, for instance, in 

 Carnivora. The meso-scapula is produced outwards as a keel, and is often developed downwards as a 

 free acromion process : when a clavicle is developed, there is a " meso-scapular segment" 3 of cartilage 

 cloven from this free process, which becomes variously metamorphosed in particular types. The 

 " pra3-coracoid " is always distal, and is always present when there is a clavicle ; and to this it is 

 variably correlated; generally, it undergoes a curious moniliform segmentation, the proximal 

 part keeping in contact with the clavicle, and the distal the counterpart of the moieties of the 

 " omosternum " of the Tadpole becoming correlated to the anterior half of the " pra3-sternum." 

 Behind these rudiments, a small distal epicoracoid often appears ; but there are only two 

 instances (for example, in the Mole, a near congener of the Monotreme, and in Man) in 

 which the coracoid mass has been seen to run through from the scapula to the Sternum. 

 There is never any " inter-clavicle," as far as I know, so that the " episternum " of the Lizard 

 is not represented ; but, as I have just shown, the so-called " episternum " of the Frog is very 

 common ; these are two very different things ! The Sternum is greatly subdivided, serially, in 

 most cases, and the " pleurostea '' are either single or symmetrical, and are separated by 

 unchanged cartilage without clefts ; by interposed fibre ; or by synovial cavities. The ossifica- 

 tion is either ectosteal or endosteal; in the latter cases it often runs rapidly into both kinds 

 afterwards, so that the ossification becomes perfect. There is sometimes a " pro-osteon " in 

 front of the keystone of the first costal arch ; this may be ectosteal or endosteal ; single or 



1 It will be seen that my descriptions of the Birds are much more condensed than those that relate 

 to the Cold-blooded types ; this is a necessity, because of my limited space. After having described the 

 main modifications in the Mammalian types, I shall treat the copious Plates illustrating the various 

 Orders as though they formed an "Atlas"; simply describing them as tersely as possible, and referring 

 the reader to the introductory remarks now to be made. 



2 This cartilaginous nucleus, although segmented from the meso-scapula, is in reality the com- 

 mencement of the prse-coracoid band, and answers to the " meso-scapular segment" and " proximal 

 prse-coracoid" of the Bird, before segmentation has taken place. 



