64 THE CERATOPSIA 



MOUNTED SPECIMENS 



Two specimens of Centrosaurus have been mounted in their entirety, both of which were col- 

 lected by Barnum Brown. Of these the first is the type of Centrosaurus nasicomus at the American 

 Museum, and the second is the specimen which forms the basis of the morphological section of this 

 memoir, Centrosaurus flexus, at Yale. 



C. nasicomus is a slab mount exposing the right side of the skeleton in the posture in which it 

 was found. C. flexus, on the other hand, is a "plastic mount," that is, the skeleton is fully articu- 

 lated and may be viewed from the left side, while the right has been clothed with a synthetically 

 constructed restoration of the animal in the flesh. 



The American Museum mount (PL IX, A) is admirably executed, but the pose is that of 

 death rather than of life, as though the creature were lying on its side with the neck flexed dorsad 

 much more than when living. The pelvic elements are also somewhat displaced, as the ischia are 

 much too near the tail to allow for the posterior outlets of the body. The feet, of course, are not 

 in their normal position. As Brown says, 39 it "differs in several important features from the Marsh 

 reconstruction of Trkeratofs and from the composite Trkeratofs skeleton mounted in the National 

 Museum. 



"The body is shorter and deeper in the posterior dorsal region, while the feet are more digiti- 

 grade with toes turning out, the axis of the manus through digit II and the axis of the pes between 

 digits II and III. 



"A Trkeratofs skeleton in the American Museum [PI. XIV, A] determines the general con- 

 struction to have been essentially the same in the two genera and the skeletal structure in Mono- 

 clonius may be considered as typical of the family Ceratopsia." 



The Yale specimen, as mounted (PI. II), differs from that at the American Museum in several 

 particulars which may be in part matters of interpretation and in part due to specific and individual 

 variation. As has been shown, not only are the usual three anterior cervicals fused into a solid 

 mass, but as a result of some pathologic condition the centra of the fifth and sixth vertebrae are also 

 coossified. The latter seem to indicate that the centra cannot have been spaced apart to the extent 

 shown in the New York mount, which precludes the possibility of the strong dorsad flexion of the 

 neck during life. This flexion is often seen in recent as well as in prehistoric animals as a result of 

 postmortem contraction of the ligaments and muscles of the neck. Hence the neck of the Yale 

 specimen is nearly straight, with the centra spaced much more nearly together. The curious back- 

 ward offsetting of the neural arch in the vertebrae of the back in this specimen has resulted in a 

 certain obliquity of the vertebral centra of this region which may or may not be correct. The sacrum 

 has been given a more nearly horizontal pose and the tail as a result does not droop at its base, 

 which, together with the position of the ischia, gives ample clearance as compared with the American 

 Museum specimen. The anterior long rib (cervical VIII) has a curious reversed curve. This has 

 evidently been placed at the posterior end of the series in the specimen at New York. The character 

 of the proximal end of this rib, however, determines its position, in addition to which the actual pos- 

 terior one is in position, fused with the ilium, both in the Yale specimen and in the Centrosaurus 

 cutleri type. The penultimate rib in both of the latter specimens is highly peculiar as it has a 

 remarkable backward curve extending through nearly half a circle. This is not shown in the type 

 of C. nasicomus or in any of the other six ceratopsian skeletons which have been mounted. It was 

 evidently a specific character. 



In placing the feet, reference was made to what may well have been ceratopsian footprints dis- 

 covered by C. M. Sternberg 40 on Peace River, British Columbia, in strata correlated with the lower 

 Blairmore of middle Lower Cretaceous age. These footprints, which are those of a quadruped, as 

 opposed to the associated tridactyl bipedal tracks, and which Sternberg calls Tetrapodosaurus 

 borealis, agree in character, in length of stride, and in width of trackway with our conception of such 



30 Brown, B., 1917, p. 300. 



40 Sternberg, C. M., 1932, p. 74, PI. V, Fig. 8. 



