20 FOSSIL REPTILIA OF THE LONDON CLAY. 



unites with its fellow from the opposite tympanum, to form a short median canal, which 

 descends backwards to the suture between the basisphenoid and the basioccipital, where 

 it joins the median canal formed by the union of the two air-passages from the back 

 part of the floor of the tympanum, which traverse the basioccipital. The common canal 

 formed by the junction of the two median canals descends along the suture to the 

 median foramen e t, fig. 2, T. VI. The air-passage from the back part of the tym- 

 panum, which traverses the basioccipital, swells out into a rhomboidal sinus in its 

 convergent course towards its fellow, and from this sinus is continued the normal 

 lateral eustachian canal, which, on each side, terminates below in the small aperture, 

 external to the* median eustachian foramen. 



That part .of the outer surface of the skull which is covered by the common 

 integument is more or less sculptured with wrinkles and pits in the Crocodilia : the 

 modifications of this pattern are shown in T. I, fig. 1, in the nilotic Crocodile, and 

 in T. VI, in the eocene Crocodile from Hordwell. The flat platform of the upper 

 surface of the cranium is perforated by two large apertures, surrounded by the bones 

 numbered 7, 8, 11, 12; these apertures are the upper outlets of the temporal fossae, 

 divided from the lower and lateral outlets by the conjoined prolongations of the 

 mastoid 8 and postfrontal 12 : if ossification were continued thence to the parietal 7, 

 the temporal fossae would be roofed over by bone, as in the Chelones. In old 

 Crocodiles and Alligators there is an approximation to this structure, and the upper 

 temporal apertures are much diminished in size. In the Gavials (T. XI, fig. 1 a) they 

 remain more widely open, and, in the fossil Gavials of the secondary strata, they are still 

 wider, as seen in T. XI, fig. 2 a ; by which the structure of the cranium approaches more 

 nearly to that of the Lacertian reptiles, where the temporal fossa is either not divided 

 into an upper and lateral outlet, or is bridged over by a very slender longitudinal bar 

 from the postfrontal to the mastoid. The lateral outlets of the temporal fossae (T. VI, 

 fig. 1) are divided from the orbits by a bar of bone developed from the postfrontal (12) 

 and malar (26), and against the inner side of the base of which the ectopte^goid 

 abuts ; the posterior boundary of the fossa is made by the tympanic (28) and squamosal 

 (27). The orbits, having the postfronto-rnalar bar (12, 26) behind, are surrounded in 

 the rest of their circumference by the frontal (11), the prefrontal (14), the lachrymal (73), 

 and the malar (26). The supraorbital or palpebral ossicle is rarely preserved in fossil 

 specimens. 



The facial or rostral part of the skull anterior to the orbit, is of great extent, broad 

 and fiat in the Alligators and some Crocodiles, narrower, rounder, and longer in other 

 Crocodiles, always most narrow, cylindrical, and elongated in the Gavials. The 

 anterior or external nostril is single, and is perforated in the middle of the anterior 

 terminal expansion of the upper jaw. This expansion is least marked in the broad- 

 headed species (compare T. VI, fig. 1, with T. Ill, fig. 1) ; in existing Crocodiles 

 and Alligators the points of the nasal bones penetrate its hind border, as at 15, fig. 1, 



