10 BRITISH FOSSIL REPTILES. 



mesiad, and overlapping the mastoid (s) and tympanic (28), is the longest, and termi- 

 nates in a point : the surface of the bone is smooth. 



The temporal fossae are broader than they are long. At their forepart the parietal 

 side-wall of the cranium expands as it advances, and is continued into the postfrontal 

 or postorbital partition. 



The orbits are rounded anteriorly, and both the upper and under parts of the frame 

 make an angular junction with the straight hinder pai't. 



The nostrils have the usual small size and backward position. 

 In both orbits some of the thin sclerotic plates of the eyeball («, s) are preserved ; 

 this is the first specimen in which I have had evidence of this structure. 



The interlocking of the teeth of the upper and lower jaws, through the singular 

 care and skill devoted by Mr. Harrison to the removal of the matrix, is peculiarly well 

 displayed in this instructive fossil. 



The foremost tooth in each premaxillary make a pair, which curve forward and 

 downward between the two foremost teeth of the lower jaw, the premaxillary teeth 

 slightly diverging as they descend, Tab. Ill, fig. 3. The succeeding premaxillary 

 teeth, four in number, alternate with mandibular ones. I cannot make out with 

 certainty the maxillo-premaxillary suture, but the fifth tooth, counting backward, seems 

 to be near to or upon it. The second premaxillary tooth is double the size of the first ; 

 the third, fourth, and fifth gradually diminish ; the sixth (first maxillary ?) is small ; the 

 seventh tooth suddenly resumes- the size of the second ; and the eighth, of nearly equal 

 size, curves down close to the seventh, and the two are interposed between the inter- 

 space of opposite mandibular teeth. From four to five smaller teeth are traceable 

 behind the eighth, and there may have been more in the upper jaw. 



Of the lower jaw ten teeth are shown on each side ; the second, third, fourth, and 

 fifth are the longest and largest, as in Tab. II. In general, the teeth of the upper jaw 

 are separated by intervals allowing the passage of those of the lower ; the teeth of the 

 foremost premaxillary pair being closer together ; and those of the foremost mandi- 

 bular pair being wider apart. They all present the usual generic character of crown — 

 long, slender, curved, pointed, circular in transverse section, with the enamel finely 

 but definitely ridged longitudinally. The longest exserted crown measures ten lines, 

 the shortest four lines, the thickness being in proportion. 



The true number of the teeth in the lower jaw is yielded by the specimen of the 

 dentary bone. Tab. Ill, fig. 2, in which twenty-five alveoli are shown on one side, and 

 twenty-four on the other. The size of the alveoli, and the extent of their interspaces, 

 are greatest at the anterior half of dentary. The small successional teeth at the 

 posterior part of the series are so advanced as to look like a double row at that part. 

 A longitudinal groove or depression at the inner side of the base of the alveoli lodged 

 the thicker mass of the vascular gum overlying the matrices of the successional teeth. 

 The skull of the Plesiosaurus dolichodeinis is broad in proportion to its length, with 



