Holland : Osteology of the Mosasaurid^e. 167 



It seems to be far more likely that they might have been a portion of the 

 sternum, located at the posterior extremity of the central axis of that 

 element, in other words constituting the xiphisternum. In that case 

 the concave surface may be supposed to have looked upward, the 

 notched end to have marked the posterior termination of the sternum, 

 and the somewhat unshapen and irregular mass of bone at the opposite 

 extremity, as shown in the Belgian specimen, to have been the sternum 

 itself. Against this view is, as has been pointed out by Professor 

 Williston, the histological character of the bone itself, so wholly 

 unlike what we know of the sternal bones as found in other animals of 

 this class, some of the remnants of the sterna of which have been 

 found in a fossil state. The bone suggests a formation in membrane, 

 rather than in cartilage. 



The other view suggested by the writer is that these bones were truly 

 lingual (glosso-hyal). The discovery of the smaller specimen lying 

 near the lower margin of the lower jaw of a specimen of Clidastes tor- 

 tor, in which there has been very little displacement of the bones of 

 the head, is suggestive. A comparison with the basi-hyoid in the 

 crocodile shows that the latter is distinctly bifurcated at its anterior 

 extremity and besides has minor lateral subdivisions, to which the 

 ridges and the intercalated depressions in the specimens we are con- 

 sidering might perhaps be regarded as morphologically analogous. 

 The bones figured by Marsh and Williston as the hyoid bones of Tylo- 

 saurns (yPlatecarpus in error) appear to the writer to be more likely 

 to be thyro-hyals than basi-hyals. There appears to be nothing im- 

 probable in the view that these great marine saurians may have had 

 tongues supported internally by an osseous framework and that the 

 posterior portion of the bones we are considering may have functioned 

 as the basi-hyoid, and the anterior bifurcated end may have func- 

 tioned as a glosso-hyal, while the bones figured by Marsh and Wil- 

 liston may have been thyro-hyals. 



