178 Prof. R. Owen on the Occurrence 



extinct marine cold-blooded air-breathers typified by the Mo- 

 sasaurus Roffmanni of Conybeare and Cuvier. 



At the date of the last two papers, quoted below *, no evi- 

 dence had been obtained, or been noticed in Prof. Cope's 

 extensive illustrations of American Cretaceous Mosasauroids i t, 

 of a sternal bone or apparatus. This most important element 

 in the question, as between the Ophidian and Lacertian nature 

 of those reptiles, has been fully and satisfactorily demonstrated 

 by the collection at Prof. Marsh's command; and as he 

 thereby feels himself justified in inferring "the presence of a 

 sternum in the entire group" J, I do not hesitate in accepting 

 this welcome addition to the complete restoration of our Leio- 

 dont modification of the Mosasaurian type ; and I beg leave 

 to offer such addition, with a few supplementary observations, 

 to the paper admitted into ' Annals and Magazine of Natural 

 History ' for July 1879. 



The following is Prof. Marsh's description of these long- 

 missing parts of the skeleton : — 



" The Sternum. 



" The absence of a sternum has been asserted by Cope 

 to be one of the important characters of the Mosasauroid 

 Reptiles § ; and this statement has been accepted by some 

 authors ||. Several specimens, however, in the Yale Museum, 

 one of which is figured in plate i. fig. 1, prove the contrary." 

 ..." The most perfect specimens of the Mosasauroid sternum 

 preserved pertain to the genus Edestosaiiras, and are of the 

 true Lacertian type. The sternum in this genus is narrow 

 and elongate in form, nearly or quite symmetrical, as shown in 

 plate i. fig. 1 [si]. [This figure is given in PL VIII. fig. 1 

 of the present paper.] It is thin, slightly concave above and 

 convex below. Its antero-lateral margins are short and 

 rounded, and have distinct grooves for the coracoids. The 

 costal margins are much longer and converge posteriorly. 

 Each has facets for five sternal ribs ; and, behind these, false 

 ribs were supported by a partially ossified pedicle, which 

 joined the end of the sternum. In the other genera of Mosa- 



* Quart. Journ. Geol. Soc. of London for November 1877, p. 682, and 

 for August 1878, p. 748. 



t ' The Vertebrata of the Cretaceous Formations of the West/ 4to, 



1875. X Mem - cit - P- 83 - 



§ 'Vertebrata of the Cretaceous,' p. 114 (1875); also 'Bulletin of 

 Survey of Territories,' p. 309 (1878). 



|| As the allegation rested on the somewhat treacherous basis of " non- 

 finding " in commonly fragmentary and more or less incomplete skeletons 

 represented by fossil remains, I limited myself to stating : — " Sternal or 

 episternal elements of the scapular arch seem not to have been recognized 

 in the American series of fossils " {toe. cit. p. 710, 1877). 



