377 



PLESIOSAURUS. 



1'LESIOSAURUS. 



373 



evidence of its soon becoming anchylosed to its fellow at the sym- 

 physis: it is chiefly remarkable for the expansion of its anterior 

 extremity. No intervening vacuity separates the angular and suran- 

 gular pieces as in the Crocodiles, but those pieces are joined together 

 throughout, as iu the Lacertian group. The surangular piece rises 

 higher and forms a sharper edge for the insertion 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 fossa*. " 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 Lacertian type. The angular piece is prolonged back- 

 wards beyond the joint, but not quite to the same extent as in the 

 Crocodiles." (Owen.) 



A tooth (lightly magnified. (Conybeare.) 



The free extremities of the Ittbs are connected together, in the 

 abdominal region, by the series of intermediate slender elongated 

 pieces, to which Conybeare gave the approximate designation of 

 stereo-costal arcs. Each of these sterno-costal arcs includes, in the 

 Plesiosaurus, seven pieces. 



8trno-CoUl Aro of Pletiouuinu. (Conybeare.) 



The broad coracoid bones, remarkably expanded as they are in the 

 antero-posterior direction, are noticed by Professor Owen as the most 

 conspicuous of those composing the Pectoral Arch : he describes their 

 internal and anterior margins as gently convex, and meeting at the 

 mesial plane, where they overlap the anterior thoracic ribs. Into 

 their anterior interspace is wedged the ento-sternal piece, consisting 

 of a short mesial process and two broad lateral expansions. A strong 

 triradiate bone, which, in Professor Owen's opinion, seems to repre- 

 sent, as in the Chelonians, the scapula and clavicle united, is arched 

 from the outer extremity of the coracoid bones, with which it com- 

 bines to form the shoulder-joint, near which last point it sends upwards 

 and obliquely backwards a branch or process representing the true 

 scapula. 



. 01. 



Humero-Sternnl part of Pltiiotaurut . (Conybeare.) 

 St., iternam ; Cl., clavicle ; Cor., coracoio>. 



The Humerus is a stout and moderately long bone, curved slightly 

 backwards, rounded at its proximal extremity, and flattened as it 

 approaches the elbow-joint. The Radius and Ulna are both short and 

 flat bones, but relatively longer and more distinctly marked than in 

 Jckthyoiawrui. The Carpus consists of a double row of small flat 



rounded ossicula, in number from six to eight. The Metacarpal Bones 

 are elongated, slender, flattened, and slightly bent. 



The Hinder or Pelvic Extremities are almost always equal, some- 

 times, as in P. macrocephalus, exceeding the anterior ones in size, and 

 the Pelvic Arch consists of a strong and short Ilium, and a broad Pubis 

 and Ischium, both of which are expanded iu the antero-posterior 

 direction analogously to the coracoid bones in the pectoral arch. 



H. 



Pelvis of Pleslosaurvs. (Conybeare.) 

 Pub., pubis ; Isch., lithium ; II., ilium. 



The radiated appenages of the pelvic arch so closely correspond 

 with those of the pectoral arch as to require little notice. The pos- 

 terior bone, or Fibula, corresponds iu its curved form with the Ulna, 

 illustrating an analogy manifested in other animals. The Tarsal Bones 

 are principally remarkable for their small size on the tibial or anterior 

 side of the series, indicating that the hind paddle had a freer inflection 

 forwards, or upon the tibia, than in the opposite direction. 



The five Metatarsals and their digits are found to correspond in 

 structure with those of the fore paddle. 





Plettosaurm tnairocephalus. (Bucklund ; Owen.) 



For the particular variations in the skeletons of the different species 

 the student should consult Professor Owen's report on the EnaUiisuiiriit, 

 in the ' Transactions of the British Association," from which the above 

 general description of the osseous parts is taken. 



The following species are recorded by Professor Owen :P. Hawk- 

 intii, Owen ; P. dolichodeirus, Conybeare ; P. macrocephalun, Cony- 

 beare ; P. brachycep/ialus, Owen ; P. macromus, Owen ; P. pachyomus, 

 Owen; P. arcnatnt, Owen; P. tvbtriyon-ui, Owen; P. trigomu, Cuv. ; 



