OSSEOUS SYSTEM OF MAMMALIA. 



301 



posterior zygapophysis in the trunk-vertebrte. The centrum, i, 

 is reduced to a compressed plate, and its hinder articular surface 

 is not more developed than is the front one of the centrum of the 

 atlas, with which it is connected by ligament. The expanse of 



Side view of cranial vertebne and appendages, Hog. 



the occipital spine, 3, has been governed by the superior develope- 

 ment of the cerebellum in the Mammalian class. 



The haemal arch of the occipital vertebra is here represented, 

 like those of the cervical vertebrae, by the pleurapophysial elements 

 only ; but these are developed into broad triangular plates with 

 outstanding processes: that called ' spine,' 5i, is exogenous; but 

 that called ' coracoid ' is developed from an independent osseous 

 centre, which is a rudiment of the haemapophysis, coalesces with 

 the pleurapophysis, and, in the present class, only attains its nor- 

 mal proportions, completing the arch at figs. 186, d, 187, 2, with the 

 haemal spine, ib. 9, in the Monotremes. The diverging appendage 

 (fore-limb, 53-57) of this arch, though retaining the general fea- 

 tures of its primitive radiated form, has been the seat of great de- 

 velopement and much modification and adjustment of its difterent 

 subdivisions in relation to the locomotive office it is now called 

 upon to perform. 



AVith the exception of this excess of developement of the 

 appendage, the defective developement and displacement of the 

 haemal arch, and the coalescence of the diapophyses in the 

 neural arch, there are few points of resemblance which are not 

 sufficiently salient between the segment represented by the bones, 

 Nr, 1 , 2, and 3, in the INIammal, and that so marked in the Fish, Vol. I. 



