178 BRITISH FOSSIL REPTILES. 



Eawlcinsii. In both species the terminal dorsals may answer to the lumbars 

 in Crocodilia; but no vertebra is ribless in Plesiosauriis until they approach in 

 position towards the end of the tail. There are no sacral vertebrae, the pelvic 

 arch being freely suspended. In PL macrocephalus, at the fiftieth vertebra 

 (from the skull), the rib descends from the neural arch upon the centrum; in 

 Pies, homolasjyondylus the change occurs in the sixty-eighth vertebra ; in aU kinds 

 the rib disappears in the terminal caudals. 



In the skull so much of the parietals are preserved as to show that the medial 

 suture persists, that they diverge, behind, to receive the superocipital, and that 

 they retain the pineal foramen near their suture with the frontals ; in these 

 characters is shown a nearer affinity to Lacertilian than to Crocodilian modifica- 

 tions of the skull. The mid-frontals extend forward to between the parial outer 

 nostrils, the interfrontal suture rising, ridge-like, to be continued into that which 

 extends forward along the nasals and premaxillaries. The nostrils open on the 

 upper surface of the skull a little anterior to the orbits, facilitating respiration 

 in the aquatic air-breather. The post-frontal is narrower than in Pies. Hawkinsii, 

 but, as in that and other Plesiosaurs, does not extend to join the mastoid. The 

 tympanic shows the strong proportions characteristic of the genus. The 

 mandible has the usual compound structure, with a coronoid eminence well 

 developed. The broad coracoid and the scapula, in position and shape 

 corresponding with the Plesiosaurian type, are well displayed on the left side of 

 the specimen ; the framework of both pectoral and pelvic fins is sufficiently well 

 preserved in the subject of Plate 17 to enable me to dispense with verbal 

 description ; it shows well-marked modifications of structure as compared with the 

 same parts in the Plesiosaurus rostratus, which most resembles Pies, macro- 

 cephalus {Sauropterygia, Plate 9) in the proportions of head and neck. 



Species — Plesiosaurus hrachycephalus, Owen (Enaliosauria, Plate 15). 



The proportion of the skidl to the cervical vertebrae, which are the same in 

 number and relative size to those in the previous species, suggested The nomen 

 specificum. The skull is also shorter in proportion to its breadth. Notwithstand- 

 ing the difference, the strength of the neck, as indicated by the processes of the 

 cervical vertebras, must have exceeded that of Plesiosaurus macrocephalus. The 

 fin-bones, as indicated by the humeri, and those preserved of the left pelvic limb, 

 were less powerfully developed. A portion of a Liassic Ammonite is preserved in 

 the mass of matrix on which the skeleton of the contemporary sea-dragon reposes. 



