S. W. WILLISTON 



at Cambridge, confirms me, in the determination of the ele- 

 ments in Brachauchenius and 

 Trinacromerum. That this was 

 not the frontal structure in all 

 plesiosaurs is also quite certain. 

 As I have stated before, the 

 suture between the frontal bone 

 and the element directly in front 

 of it had never been positively 

 determined. A suture has been 

 given for the skull of Plesiosaurus 

 macrocephalus, nearly transverse 

 in position and immediately in 

 front of the pineal opening, but 

 this is incorrect. However, an 

 isolated specimen of the frontal 

 region of a species of Plesiosaurus 

 in the British Museum, for the 

 privilege of examining which I 

 am indebted to Dr. Woodward, 

 shows clearly a fronto-parietal 

 suture, beginning some distance 

 in front of the pineal opening, 

 in the middle line, and extend- 

 ing obliquely outward and for- 

 ward, clearly distinguishing a 

 median frontal bone; and the 

 one at either side of this true 

 frontal is as clearly the prefrontal. 

 In the later plesiosaurs, or some 

 of them, I beheve that the pro- 

 longation of this parietal projec- 

 tion forward to the backwardly 

 produced premaxilla has separ- 

 ated the real frontals; and this 



interpretation is confirmed by the examination of the bones of this 



region in T. anonymum described farther on. 



