Holland : Osteology of the Mosasaurid^e. 163 



the osteology of the group and to indicate to me where the bones, in 

 his judgment, ought to be placed. In a letter written by him on Jan- 

 uary 13, 1907, he says : 



"It is rather a humiliating confession to make, after thirty years' 

 acquaintance with the Mosasaurs, to say that your bone ' stumps ' me, 

 but it is a fact. The lingual bone has been figured by Marsh for 

 Tylosaurus, a copy of whose figure you will find in Volume IV of the 

 University Geological Survey of Kansas, Plate XXXI, Figures 1-3, 

 there wrongly ascribed to Platecarpus. Somewhere else I make men- 

 tion of the hyoid bones of Platecarpus being very much like the fig- 

 ures, having found specimens at a later date. Mosasaurus and Cli- 

 dastes are scarcely distinct generically, and they differ in so many ways 

 from Platecarpus and Tylosaurus that one would expect to find some 

 differences in the hyoids, but scarcely so great differences as your bone 

 would indicate. 



" The sternum of no Mosasaur is ever really ossified ; the bones of 

 the sternum are frequently preserved as calcified cartilage, quite of the 

 consistency and structure of the sternal ribs. As your bone is really 

 ossified, I mean, having the true bone structure, it cannot be a sternal 

 element. The interclavicle, or episternum, whichever one chooses to 

 call it, has so far been found in Plioplatecarpus, Platecarpus, and Hol- 

 osaurus, that is, in Mosasaurs of the Platecarpus type. I have sus- 

 pected in the past that its retention was rather characteristic of this 

 group, but there are really no good reasons why it should not be pre- 

 served as a vestige in the Mosasaurus type also, though it has never 

 been found in that genus or in Clidastes. 



"The bone has never been figured for any genus. One of my 

 students 1 has prepared a paper on the girdles and limbs of Holosaurus, 

 which is very doubtfully distinct from Platecarpus, and has included 

 the figure of an interclavicle from a specimen in which there can be 

 no doubt of its identity, having been found in position between the 

 coracoids. I have made a photograph of his figure for you and enclose 

 it herewith. You see that it is a mere vestige, a slender flattened rod 

 of bone, almost ' without form and void.' In Platecarpus I have 

 found the bone rather better developed, and apparently with facets 

 for vestigial clavicles. Your bones show material differences from 



l S. R. Capps, Jr., "The Girdles and Hind Limb of Holosaurus abrupt us 

 Marsh," Journal of Geology, Vol. XV, No. 4, pp. 350-356, May, 1907. (See 

 Fig. 1.) 



