BRITISH FOSSIL REPTILES. 55 



formation of the circumference of the orbit below the lachrymal ; 

 and the malar bone rests by an oblique suture upon its posterior 

 extremity. The posterior margin of the malar bone is joined to 

 the posterior frontal as well as to the zygomatic bone, and thus 

 completes the osseous boundary of the orbit posteriorly. 



Lower jaw. — The lower jaw of the Plesiosaur presents the 

 complicated structure usual in the Saurian order. The dentary 

 piece appears soon to become anchylosed to its fellow at the 

 symphysis, and is chiefly remarkable for the expansion of its 

 anterior extremity. The angular and surangular pieces are not 

 separated by an intervening vacuity as in the Crocodiles, but 

 are joined together throughout as in the Lacertian group. The 

 surangular rises higher and forms a sharper edge for the inser- 

 tion of the temporal muscles than in the Crocodiles, a structure 

 which agrees with the greater development of these muscles, as 

 indicated by the size of the temporal fossae. The articular piece 

 presents a regular and deep concavity for the reception of the 

 articular end of the tympanic bone : it is, as Mr. Conybeare has 

 well remarked*, more developed than in the Crocodile, and thus 

 approximates more nearly to the corresponding part in the La- 

 certian type. The angular piece is prolonged backwards beyond 

 the joint, but not quite to the same extent as in the Crocodiles. 



Sterno-costal arcs. — The ordinary or vertebral ribs have been 

 already spoken of as essential parts or appendages of a vertebra : 

 their free extremities are connected together, in the abdominal 

 region, by a series of intermediate slender elongated pieces, 

 termed by Mr. Conybeare the 'sterno-costal arcs.' Each arc 

 includes, in the Plesiosanrus, seven pieces : the median one is 

 transversely elongated, slightly bent, and pointed at both ex- 

 tremities ; the lateral pieces have a similar form, except that the 

 extremity of the outermost, which joins the vertebral rib, is 

 obtuse : each piece as it recedes from the median line overlaps 

 the anterior part of the one which it succeeds, where it is 

 adapted to an oblique groove. This kind of joint probably 

 admitted of a yielding or sliding motion of the pieces one upon 

 the other, and favoured, as Dr. Buckland has observed, con- 

 siderable expansion of the cavity containing the lungs. 



Pectoral arch. — Of the bones composing the pectoral arch 

 the broad coracoids are the most conspicuous on account of their 

 remarkable expanse in the antero-posterior direction ; their in- 

 ternal and anterior margins are gently convex, and meet at the 

 mesial plane, where they overlap the anterior thoracic ribs. 

 The ento-sternal piece is wedged into their anterior interspace ; 



* Geol. Trans., 1822, p. 121. 



