72 REPORT— 1839. 



I have not met with any specimens from the Dorsetshire or 

 Street lias referrible to this species. 



Plesiosaurus macromus. 



In the PL doUchodeirus and Hawkinsii, which may be re- 

 garded as the typical species of the present most singular genus, 

 the anterior and posterior paddles are of equal size. The PL 

 macrocejihalus and bracJiycephalus are distinguished in addition 

 to other characters by the superior length of the hinder paddles. 

 In the present species the contrary proportions prevail ; here vre 

 lind at length an instance in which the Plesiosaurus resembles 

 the Ichthyosaurus, in the superior size of the anterior as com- 

 pared with the posterior extremities ; and the equality of the 

 locomotive members, as respects their length, proves to be a 

 specific and not a generic character. A considerable proportion 

 of the skeleton of the PL macromus was discovered by Miss 

 Anning in the lias near Lyme, and now forms part of the valu- 

 able collection of Miss Philpotts. These interesting remains 

 include the greater part of the vertebral column, with tlie prin- 

 cipal bones of the anterior and posterior extremities ; but the 

 skull and teeth are unfortunately wanting. 



The cervical vertebrae resemble those of the PL doUchodeirus 

 in their general form and proportions, and in the relative posi- 

 tion of the neurapophysial and costal surfaces in the anterior 

 cervical vertebras, but differ in the character of the articular 

 surfaces of the centrum. The body of one of the vertebra? from 

 the middle of the neck gives the following admeasurements : 



Inch. Lines. 

 Length or antero-posterior diameter. . 1 4 



Vertical diameter 1 4 



Transverse diameter 1 6 



The anterior and posterior articular surfaces are gently and 

 uniformly concave, without the central rising described by Mr. 

 Conybeare in the PL doUchodeirus, and which is present in 

 some other species. In many of the vertebrae of the present 

 specimen there is a transverse linear impression in the centre 

 of the above-mentioned articular surfaces. The lateral surfaces 

 are sculptured at the anterior and posterior margins by nume- 

 rous longitudinal irregular grooves ; the intermediate surface 

 is comparatively smoother. A narrow vertical ridge is con- 

 tinued from the descending angle of the neurapophysial de- 

 pression to the middle of the upper costal surface. The di- 

 stance which intervenes between the neurapophysial and coetal 



