A lELEOSAtTE PROM THE WHITBY LIAS, 631 



18 millimetres. Over the posterior part of the brain the thiekness 

 of the bone was more than doubled. The section shows the basioc- 

 cipital to be large and strong, and very closely blended with the basi- 

 sphenoid. There is no separate suture indicating the presphenoid. 

 In the median line, and in the basisphenoid bone, below and some- 

 what behind the pituitary excavation, there is an inverted flask- 

 shaped cavity 18 millimetres wide, which terminates downward in a 

 tube nearly 1 centimetre wide, which is directed backward and im- 

 perfectly preserved, and appears almost to impinge upon the basi- 

 occipital bone. This tube (Eu) I regard as the Eustachian tube ; 

 and the ovate cavity with which it communicates (TA) T regard as 

 being a portion of the tympanic air-cell, which would thus appear 

 to have no bony separation dividing the portions on the two sides 

 of the head. This tympanic air-cell on the right-hand section 

 has become more irregular in shape, and is larger, being about 

 2| centimetres deep and about a centimetre high, though a bony 

 process indents the posterior margin considerably. The fractured 

 condition of the external part of the right half of the brain- 

 cavity shows another section of the tympanic air-cell, still irregular 

 in form, but subquadrate and enlarging as it passes outward ; while 

 the vertical fracture parallel to the median line of the skull, seen 

 on the outer part of the left side of the skull, shows this air-cavity, 

 which has a thin border formed in front by the prootic bone of 

 Huxley (alisphenoid bone of Owen), to be subovate, rather irregular 

 in outline, about 4| centimetres long and 3| centimetres deep, 

 with a sharp plate of bone penetrating inward from the middle of 

 its posterior side. Anterior to this cavity (which attains a develop- 

 ment altogether unparalleled among the living Crocodilia), the left- 

 hand side of the median section of the skull shows an ovate per- 

 foration IJ centimetre in advance of it. This cavity is 2| centi- 

 metres long and less than 1 deep; its border is somewhat irre- 

 gular, and I regard it as a section of the carotid canal (c c). It 

 lies just below and on one side of the position of the pituitary pit ; 

 and appears to be a canal which passes still further outward as it 

 extends forward. The basioccipital has much the same form as in 

 Plesiosaurs. The occipital condyle is rounded, 4| centimetres deep 

 and about 5| wide ; it has a central depression, and is margined by 

 a sharp ridge where the external articular surface ends. This ridge 

 increases in sharpness as it descends ; and there is a deep excavation 

 or constriction under its inferior border, so that from the neural canal 

 to the base in front of the ridge the depth is 3| centimetres ; and 

 here the excavation is concave. The bone extends downward and a 

 little backward in the median line, on the side of which a nutritive 

 foramen enters the bone. The texture of the bone is dense pos- 

 teriorly towards the articular surface, and the ceUs are arranged 

 longitudinally ; but in the anterior part of the bone the cells become 

 relatively very large. The length of the bone cannot be certainly 

 stated, but appears to be about 5| centimetres ; its anterior border 

 is convex from above downward. The basisphenoid (Bs) is densest 



