154 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



features of a beautifully preserved skull of 3I<>ro.sauru.'< (jrandix. now in 

 the American Museum of Natural History, New York. 



Although Marsh determined most of the elements composing the 

 posterior aspect (see fig. 1) of the skull of Morosau7'us agilis^ he did 

 not describe them in detail; therefore the writer, because of the rarit}^ 

 of the Opisthocoelian dinosaurian skulls, as well as to make our 

 knowledge of these crania a little more exact, now attempts such a 

 description. 



DESCRIPTION OF SKULL. 



The skull of M. agilis, although somewhat distorted by crushing, 

 shows quite clearly the relationship of the several elements of the pos- 

 terior portion. The occipital segment consists of four bones, basioc- 

 cipital, exoccipitals, and supraoccipital, and the plane of the occiput 

 forms an obtuse angle (see Plate XII) with the f ronto-parietal part of 

 the skull. The basioccipital extends well posterior "to the median por- 

 tion of the skull and forms the greater part of the subcircular convex 

 condyle. Dorsally it unites by suture with the exoccipitals (see fig. 

 3). The latter bones enter into the formation of the occipital condyle 

 and entirely exclude the basioccipital from the boundary of the fora- 

 men magnum. This arrangement of the occipital bones approximates 

 the conditions found in the Chelonia. Atlantosaurus montanus, as 

 figured" by Marsh, shows a similar arrangement of these elements. 



The ventral side of the basioccipital in advance of the cond3de is 

 somewhat concave longitudinally and convex transversely, thus form- 

 ing a well-defined neck. Anterior to this constriction a broad hypo- 

 physis begins to develop, being directed downward. Ventrally, how- 

 ever, this region has been badh' injured, and the greater part of the 

 basioccipital processes are missing. The suture between the basioc- 

 cipital and basisphenoid is entirely obliterated. 



The en'occipitals are not as broad nor as strongly developed as in 

 DiplodoGus, and they also differ in the backward extension of the basal 

 portion for articulation with the basioccipital. The exoccipitals entirel}^ 

 enclose the foramen magnum and also enter into the formation of the 

 occipital condyle. Laterally they articulate with the parietals and 

 squamosals, the articulation with the latter being principally with the 

 paraoccipital process which extends outward, backward, and down- 

 ward. This process is expanded medially, but tapers to a rounded 

 obtuse end. There are no posterior fossa in the skull of Mor-osauriis 

 agilis. The suture at the union of the exoccipitals is entirely closed. 



The supraoccipital is roughh' an irregular subtriangular plate of 

 bone occup3'ing the space between the parietals above and the exoc- 

 cipitals below. On the median posterior surface a very pronounced, 



«0. C. Marsh, Dinosaurs of North America, Sixteenth Ann. Rept., U. S. Geol. 

 Survey, Pt. 1, 1896, pi. xv, fig. 1. 



