136 PEOF. OWEN OX GENEEIC CHAEACTEES 



the glenoid cavity. In Plesiosaurus the hinder end of the scapula, 

 which is the thickest part of the bone, is thus divided pretty equally 

 between its coracoidal (ch) and humeral (h) articular surfaces, both 

 being rough or " syndesmosal." In advance of the surfaces h, ch, 

 the scapula thins and contracts, chiefly by a strong margino-mesial 

 concavity, contributing the outer border of the " coraco-scapular 

 vacuity " (jb. ib., cs). The outer or lateral and thicker border of 

 the scapula is nearly straight ; and the bone extending forward and 

 slightly mesiad, expands to unite with the episternurn, 59, by the 

 suture, sh. 



The episternurn, 59, presents anteriorly a mesial notch, from each 

 angle of which the bone extends outward and backward to its 

 sutural union, sh, with the fore end of the scapula. At this union 

 the episternurn contracts, and is continued backward to join the 

 coracoids, passing a short way internal to (" centrad" of) them, and 

 appearing outwardly as a terminally pointed portion, ms, at the fore 

 part of the narrow mesial interspace of the coracoids, 52*, which 

 interspace interrupts anteriorly their extensive mesial suture with 

 each other. 



Thus the sterno-coraco-scapular frame, or mass, presents an 

 anterior and a posterior emargination and a pair of subcircular 

 vacuities. The above-defined characters of this portion of the 

 skeleton, save that of the scapular element, are common moreover 

 to both the generic groups of the Sauropterygia. 



The chief and suggestive modification of the mass in the Plio- 

 saurian genus is the retention of a typical character of the scapula 

 which is lost in the more modified or specialized Plesiosaurian 

 forms, viz. the production of the part of the blade-bone (fig. 2, 51*), 

 laterad and dorsad, where it terminates freely f. This portion re- 

 presents the main body of the scapula in the higher Vertebrates, 

 but, as in the " Allantoic group " (Eeptilia and Aves) J, without 

 expanding. 



The portion of the scapula, 51, common to both genera, which 

 contributes its share (figs. 1 & 2, h) to the glenoid cavity, is separated 

 in Pliikaurus from the free portion, 51*, by the notch, n. In ad- 

 vance of this the Pliosaurian differs from the Plesiosaurian scapula 

 by its greater relative breadth, extending its sutural border, sh y 

 mesiad, so as to touch or join the fore end of the coracoid, 52*. 



The coracoids retain their large proportional size, but have a less 

 even or flattened outer surface ; mesially they bulge to their common 

 suture, s, giving more room to the ventral or visceral cavity ; and, at 

 the transverse margin parallel with the hind border which they con- 

 tribute to the vacuities, cs, cs, they bend dorsad, suddenly contract, 



f This character I added to the generalized illustration of the Sauroptery- 

 gian skeleton in my ' Palaeontology,' p. 227, fig. 71. 



I "In their generation and development modern Batrachians differ from 

 other cold-blooded air-breatbers and agree with fishes. Birds, by genetic and 

 developmental characters, as well as by the general plan of their organization, 

 are more intimately and naturally allied to the oviparous Saurian s than to the 

 viviparous Mammals." — Anat. of Vertebrates, v^ol. i. 1866, pp. 6 and 7. 



