242 Geological Society. 



gated the plesiosaurian neck. He compared the chelonian plastron 

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

 in the chelonian type upon the potential energy due to the condi- 

 tions of locomotive activity, and used this consideration in elucida- 

 tion of chelonian and plesiosaurian resemblances and differences. 

 The plesiosaurian interclavicle he regarded as homologous with the 

 chelonian 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 be- 

 lieved that the clavicles were ocasionally distinct from the inter- 

 clavicle, though usually blended with it, just as all these ossifica- 

 tions 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 PUsiosaurus was divided into two families — the 

 Plesiosauridas, containing the genus PUsiosaurus, and the Elasmo- 

 sauridas, with Eretmosaurus, Oolymbosaurus, and Muramosaurus. 



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

 distinct clavicles. Eretmosaurus has neither clavicle nor inter- 

 clavicle ; and the scapulae, concave in front, are blended in the median 

 line, and blended laterally with the coracoids. Its type is PUsio- 

 saurus rugosus of the Lias. Colymbosaurus has for its type PUsio- 

 saurus megadeirus of the Kimmeridge Clay. It has no interclavicle; 

 the scapulas are prolonged forward in a wedge, and backward so as 

 to meet the coracoids in the median line and enclose two coraco- 

 scapular foramina. 



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

 It has no interclavicle ; but the scapulas 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 

 basisphenoid 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 vertebras 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 zygapo- 



