378 Reports and Proceedings — 



gated the plesiosauriaii neck. He compared tlie chelonian plastron 

 with the plesiosaurian girdle-bones, to show the dependence of form 

 in the chelonian type upon the potential energy due to the conditions 

 of locomotive activity, and used this consideration in elucidation of 

 chelonian and plesiosaurian resemblances and differences. The 

 plesiosaurian interclavicle he regarded as homologous with the che- 

 lonian and lacertian interclavicle ; but the chelonian clavicles he 

 believed to be epiphysial, potential representatives of the precora- 

 coids, and therefore bones of which no analogues should be sought 

 in Plesiosaurs on the theory of their chelonian affinities. He 

 believed that the clavicles were occasionally distinct from the inter- 

 clavicle, though usually blended with it, just as all these ossifications 

 become obliterated by scapular extension. 



The restorations and interpretations of the plesiosaurian pectoral 

 girdle given by Conybeare, Hawkins, Owen, Huxley, Cope, and 

 Phillips, were discussed, and reasons given for dissenting from their 

 views. The old genus Plesiosaurus was divided into two families, 

 the Plesiosauridse, containing the genus Plesiosaurus, and the Elasmo- 

 saurida?., with Eretmosaiirus, Colymbosaurus, and Murcenosaurus. 



A new type was taken for the genus Plesiosaurus, which showed 

 distinct clavicles. Eretmosaiirus has neither clavicle nor interclavicle, 

 and the scapulas, concave in front, are blended in the median line, 

 and blended laterally with the coracoids. Its type is Plesiosaurus 

 rugosus of the Lias. Colymbosaurus has for its type Plesiosaurus 

 megadeirus of the Kimmeridge Clay. It has no interclavicle, the 

 scapulse are prolonged forward in a wedge and backward, so as to 

 meet the coracoids in the median line, and enclose two coraco-scapular 

 foramina. 



Murcenosaurus is founded on a new type from the Oxford Clay. 

 It has no interclavicle, but the scapula are prolonged forward to 

 meet in the median line ; they are not prolonged backward to meet 

 the coracoids, hence but one coraco-scapular foramen is formed. A 

 similar condition marks the pelvic girdle. 



2. "Murcenosaurus Leedsii (Seeley), a Plesiosaurian from the 

 Oxford Clay."— Part I. By Harry G. Seeley, Esq., F.L.S., F.G.S. 



All parts of the animal, except teeth, ribs, and hind limbs, were 

 described. The premaxillary bones extend bird-like between the 

 nares to the frontals. The foramen parietale is between the parietal 

 and frontal, and directed backward. The cerebral lobes of the brain 

 have a chelonian form, are prolonged in olfactory nerves, like those 

 of Teleosaurus, and have the optic lobes moderately developed. The 

 exoccipital bones do not enter into the occipital condyle. The basi- 

 sphenoid is perforated by the carotids, as in Ichthyosaurus. The 

 hypoglossal nerve does not perforate the exoccipital bone. 



There are 44 cervical, 3 pectoral, 20 dorsal, 4 sacral, and the 

 first 8 caudal vertebrse preserved. Atlas and axis are anchylosed. 

 The zygapophyses are semicylindrical, being concave in front and 

 convex behind. A process of the neural spine is prolonged between 

 the anterior zygapophyses, so as to divide the posterior zygapophyses 

 and lock between them. The caudal vertebrse have the facets for 



