Mr. T. Atthey on Pteroplax cornuta. 373 



probably absent. The height of the occipital surface near its 

 middle is -^ inch ; at its outer part at the base of the horns, 

 where the bones are very strong, and have been able to resist 

 pressure better than other parts, it is yV inch. 



Upper surface of the larger cranium. — Tliis has not been 

 figured. Having been carefully cleared of the matrix it 

 shows all the sutures ; but the bones are badly preserved and 

 the pittings almost all obliterated. The following are its 

 dimensions : — from the median line of the anterior broken end 

 to the point of the epiotic horns 7 inches, from the same point 

 along the median line to the posterior edge of the occiput 5^ 

 inches, from the middle point of occipital border to point of 

 left epiotic horn 1\ inch, between the epiotic horns 'df^ inches. 

 The breadth of the skull at the occipital margin 3j\j- inches, 

 over the posterior margin of the parietals 3 % inches, between 

 the posterior points of the orbital curve 3^ inches, between 

 the anterior points of the same IfL- inch ; the breadth of the 

 broken anterior end of the frontals is nearly one inch. 



The right epiotic horn is broken off at the end ; otherwise 

 the outline of this cranium is on the whole nearly the same as 

 that of the smaller specimen ; it is rather broader, and perhaps 

 belonged to an older animal : the frontals are worn as it were 

 in front ; but the right side appears to have the normal length, 

 whilst the left is shortened. 



Under surface of the larger cranium (PI. XIII. fig. 1). — In 

 this the entire under surface of the cranial vault can be seen ; 

 all the parts below are gone ; the median suture of the vault 

 throughout with the parietal foramen is visible ; the palate, 

 maxillary, and premaxillary bones are wanting, and may have 

 been more or less cartilaginous or loosely attached ; tlie pre- 

 sphenoid and sphenoid median ridge and the basioccipital, 

 which are seen in the skulls of Loxomma and A)ithracosaurus, 

 are also gone. The quadrates at the base of the epiotic horns 

 are the only bones of the base of the skull that remain. The 

 horns are a good deal damaged. The articular condyle for the 

 mandible is not to be seen. The under surface of the frontal 

 bones is grooved on each side of a median ridge, along their 

 whole length, as if they had formed the roof of a double nasal 

 cavity extending from the snout to the throat. 



The parietal foramen is larg(3, o])cn, and funnel-shaped, 

 widening out greatly as it passes through the thickness of the 

 cranium ; it is here j\ inch long and -^ inch broad, wiiilst on 

 the upper sm-face of the skull it is only 5 inch long by -^\ inch 

 broad. Its margin is surrounded, exccj)t in front, by a sharp 

 ridge of bone, from which p;iss off laterally smaller ridges, 

 which, dividing, enclose small smooth depressions that extend 



