13 



Plate II. fig. 1, gives a front view of the fifth segment of the dorsal or thoracic 

 region of the trunk : it deviates from the archetype inasmuch as the neurapophyses, 

 n, n, have coalesced, as in other mammals, with the centrum, c, below, and are connate 

 with the neural spine, ns, above : the haemal arch is also vastly expanded in relation 

 to the greatly developed vascular centres which it was destined to encompass ; the 

 pleurapophyses, pi, pi, being elongated and bent down, and the hsemapophyses, h.h, 

 removed from the centrum and articulated to the ends of the pleurapophyses, and, 

 by a double synovial joint, .yV, to the ha?mal spine or sternal bone h.s. 



The coalesced centrum and neural arch constitute the so-called 'dorsal vertebra,' 

 and the one selected is the fifth of that series counting backwards. 



The centrum, c, or body of the vertebra, is wedge-shaped, with its base upwards, 

 forming the floor of the capacious neural canal, and the sides — slightly concave 

 lengthwise, almost flattened vertically — converge to the inferior surface, which is 

 formed by an obtuse ridge : the centrum expands slightly at its articular ends, and 

 so that the contour of the anterior one is rather oval than trihedral ; this surface is 

 slightly depressed at its middle, slightly convex in the rest of its extent ; the posterior 

 articular surface is larger than the anterior one and flatter, but is also a little 

 depressed at the middle: the two upper angles of the hinder end, probably con- 

 tributed by the neurapophyses in the development of the vertebra, are slightly pro- 

 duced and truncate, ottering each a smooth, flat, small subcircular surface, c', for 

 the head of the rib of the preceding vertebra; the corresponding part of the rib of 

 the present segment is marked c". The neurapophyses, n, n, rise each by a slender 

 base which has coalesced with the anterior half of the upper and outer angle of the 

 centrum : they diverge from each other and expand as they rise ; then, developing 

 some articular surfaces and exogenous processes from their outer surface, arch 

 towards each other, increasing rapidly in antero-posterior extent, and coalesce above 

 the neural canal ; where they support the zygapophyses, z, z, and the thick and strong 

 neural spine IM. The roof of the neural arch, thus formed, projects some way beyond 

 the anterior surface of the centrum, and extends almost to the posterior surface. 

 The inner surface of the neural arch is as remarkable for its even smoothness, as the 

 outer surface is for its various prominences and depressions. The outer side of the 

 basal half of the neurapophysis supports a large elliptical articular surface (n!), con- 

 cave from above downwards and backwards : the overhanging fore-part of the arch 

 supports the two flat oval anterior zygapophyses, z, z, the articular surfaces of which 

 look almost directly upwards ; on the under surface of the back part of the arch are 

 the two posterior zygapophyses looking almost downwards, and between these is a 

 rough longitudinal prominent ridge. The neural spine is moderately long, subcom- 

 pressed and snbtrihedral, with a sharp anterior margin, smooth sides, and a rough 

 thick posterior surface, developing a median longitudinal ridge: the summit expands 

 into a rough triangular almost flattened surface. 



For the convenience of describing and comparing the different exogenous processes 



