28 BRITISH FOSSIL REPTILES. 



anterior tooth being about to be shed. The right side of the symphysial part of the 

 jaw (Tab. IX, fig. 2) contains nine teeth, the third, fifth, and seventh being the largest. 

 The fifth (Tab. IX, fig. 5) measures 2§ inches in length of crown, following the curve. 

 The fourth and sixth teeth were emerging from sockets larger than themselves, and 

 arc successional teeth. The unusual length of the symphysis corresponds with the 

 prolongation of the premaxillary above. 



In the same locality and formation with the skeleton above described was discovered 

 the portion of skull, corresponding in size, and, so far as it is preserved, in shape with 

 that of the skeleton of the Plesiosaurus rosiratus, to which species, therefore, I pro- 

 visionally refer the specimen (Tab. XIII). It includes the basi-occipital, right ex- 

 occipital, basi-sphenoid, portions of pterygoids, ecto-pterygoids, palatines, with fragments 

 of the maxillary and of the right ramus of the mandible. In the figures of this specimen 

 (Tab. XIII) the mandibular part is omitted. The left ex-occipital is wanting ; the 

 right (ib., 2) is displaced, so that the whole of the upper part of the basi- 

 occipital (ib., 1) is exposed. This shows that the bases of the ex-occipitals are divided 

 from each other by a narrow tract, and that the basi-occipital forms the whole of the 

 condyle and of the median part of the floor of the epencephalic compartment of 

 the cranium; this part of the bone (1) measures across its most contracted portion 

 about three lines. The condyle is subhemispheric, ^Yith the transverse diameter 

 rather the longest, and with a slight and irregular depression a little above its centre. 

 The upper border of the condyle (fig. 1) is on a level with the advanced epencephalic 

 surface (1) of the basi-occipital, from which it is divided by a shallow and narrow 

 transverse channel; the lower border (fig. 2, o) projects abruptly downward, and is 

 divided from the more advanced surface of the basi-occipital by a transverse furrow, 

 three lines wide and four or five lines deep. The sui'face of the basi-occipital is covered 

 by the posterior border of the pterygoids (24) which underlie it, extending backward, 

 so as to leave only parts of the pair of rough and tuberous basi-occipital processes {h,h) 

 exposed. The posterior part of the epencephalic surface of the basi-occipital 

 (Tab. XIII, fig. 1, 1) is smooth and concave transversely; but, as it advances, it 

 becomes irregular and expands, and apparently is divided by an irregular protuberance 

 at the middle part. The neurapophysial surfaces {n,n) on each side are, as in the 

 succeeding centrums, triangular, with the angles rounded ofi". About a line in advance 

 of these is the 'harmonia,' or straight suture, indicating the flat synchondrosis by 

 which the epencephalic unites with the mesencephalic centrum (basisphenoid, ib., .J), thus 

 repeating the kind of union which attaches the centrum of the atlas to that of the axis 

 ' vertebra;. The neural surface (5) expands to a greater breadth than it had attained 

 in the basi-occipital, measuring fourteen lines across ; it has a smooth, undulating 

 surface, moderately concave in the middle, where it sinks below the level of the 

 epencephalic surface (1). The sides of the mesencephalic surface are bounded by 

 a narrow ridge, seemingly an exogenous growth from the centrum, as in some modern 



