Prof, Owen on the genus Dicynodon. 63 



floor of the cranial cavity before forming the articular convexity, which constitutes 

 no immediate part of the circumference of the foramen magnum (m). The shape of 

 the foramen itself in the Chelonians is more like that of the Dicynodon than it is in 

 the Lizards. In the Chelonians the lower three-fourths of the foramen is formed 

 by the ex-occipitals ; in the Crocodilians the upper three-fourths. In the Dicyno- 

 don the basi-occipital (1) forms a very small part of the lower border of the foramen ; 

 the ex-occipitals (2) constitute the rest of its lower half, and it is completed above 

 by the broad and high supra-occipital (4). In the Crocodilians this element is 

 excluded from any share in the formation of the foramen magnum. The Dicy- 

 nodon, therefore, in the essential anatomical characters of the occipital region 

 of the skull, and in its shape, chiefly due to the angle formed by the overhanging 

 parieto-mastoid ridge, adheres to the Lacertian type ; but in the extent and ossi- 

 fication of this region it resembles the Crocodilia. It is not, however, devoid of 

 characteristics peculiar to itself : the broad semicircular lower half of the occiput 

 sends out a series of short and thick, equidistant processes, like rays. Two 

 (/', V) are continued downwards and a little outwards from the infero-lateral 

 parts of the basi-occipital : above these the longer and stronger par-occipital 

 processes (3, 3)* extend outwards and a little downwards to abut against the 

 lower ends of the tympanic pedicles : the next process (2') above is a narrower 

 and shorter one, which projects from the upper and outer angle of the par- 

 occipital into the interspace separating it from the tympanic bone ; but, appa- 

 rently, without effecting a junction with that bone : the fourth radiating pro- 

 cess (4') on each side forms the lower prominent angle of the triangular supra- 

 occipital plate, and abuts against the mastoid bone. The condyloid foramen (2") 

 is large, and is pierced on each side of the single condyle ; not, as in the Croco- 

 diles, above that prominence. 



In commencing the examination and comparison of the upper region of the skull 

 of the Dicynodon (PI. III. fig. 2.), we cannot fail to be struck with the very con- 

 tracted form of the cranium proper (8, 8'), which is scarcely more than two-thirds 

 the breadth of the inter-orbital space. It reminds one of the lowest Batrachians ; 

 but the temporal vacuities {t, t), on each side, are divided by a strong bridge of 

 bone from the orbits, which is not the case with the Batrachians. This trans- 

 verse bar or bridge is chiefly formed by the post-frontals (11, 11), which are not 

 produced backwards, as in most Lizards : neither do the mastoids (7, 7) join the 

 post-frontals to overarch any part of the temporal spaces, as in the Crocodilians ; 

 but they are confined to the posterior circumference of those spaces, which are 

 bounded or over-arched laterally by a single bridge, formed chiefly by the zygomatic 



* These {occipitaux externes, Cuv.) are not divided by a suture from the ex-occipitals (occipitaux late- 

 raux) as in the Chelonia. 



k2 



