158 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxu. 



posterior surfaces of the exoccipitals with which they articulate. A 

 notch and groove on the anterior end and side appear to indicate the 

 course of a nerve. The internal surface of this bone is gently concave 

 antero-posteriorly. The shaft gradually converges from the thick 

 anterior to a thin posterior end. 



Marsh has found the pro-atlas present in two members of the 

 Ojpisthoeoelia {Sauropoda), i. e., Morosaurus und Apatosau7'us {Bnmto- 

 saurus)."- They have not been found in Diplodocus^ although, after 

 an examination of the posterior part of two skulls in the collection of 

 this museum, a flattened roughened surface on the exoccipitals just 

 above the foramen magnum would appear to indicate that these bones 

 were also present in this genus. 



The 'pro-atlax is often regarded as a vestige of a degenerate vertebra 

 4n front of the atlas, but Re^^nolds* says in the CrocodUla it is a mem- 

 brane bone, and therefore not properly a verte- 

 bral element. In the crocodile these lateral 

 pieces are united on the median line, thus form- 

 ing a single element. 



Marsh has designated these elements in M. 

 agilis as the ""post occipital bones,'' but as 

 FIG. 4.-R1GHT HALF OP THE ^^ appcar homologous with the dorso-lateral 



PRO- ATLAS OF MOROSAURUS . 



AGILIS (Cat. No. 6384), a nat. elements in Rhynchocephalia, certain Lacertilla^ 

 SIZE, a, LATERAL VIEW; 6, CrocodaUt., Pteromuvia, and Chelonia, to the 



anterior end of same. -I ■ •! 1 



writer there seems no good reason why the 

 older term, pro-atlas, should not apply here. 



Marsh gives the following measurements of the pro-atlas of Moro- 

 saiirus grandis: 



mm. 



Greatest length 65 



Greatest length of surface opposed to exoccipital 30 



No. 5384. Greatest length proatlas of M. agilis 45 



No. 5384. Greatest length of surface opposed to exoccipital of 31. agilis 27 



Atlas. — The atlas is composed of four separate pieces, the inter- 

 centrum, two neural arches or neurocentra, and the odontoid process. 



Like the atlas of Diplodocus^ this element in Morosauriis agilis is 

 short antero-posteriorly and without transverse processes. The pieces 

 composing the atlas were found articulated, and although the}^ have 

 suffered somewhat from lateral compression (see fig. 5), there was not 

 enough displacement to render it difticult to properly interpret them. 



The intercentrum is, roughl}", a moderately thick subcrescentic bone, 

 both ends being truncated almost horizontally, thus forming two sur- 

 faces for articulation with the neural arches. The upper anterior 

 surface is deepl}' excavated and well adapted for articulation with the 



«E. S. Riggs, Field Columbian Museum Publication, p. 82, Geol. Ser., II, No. 4, Aug. 

 1, 1903. Riggs has shown that Brontosarinis is a synonym of Apati).'«mr(is. 

 I'H. H. Re.ynoids, The Vertebrate Skeleton, 1897, p. 240. 



