7i6 5. W. WILLISTON 



Trinacromerum hentonianum Cragin 

 Skull. — In his original description Cragin speaks of three skulls 

 referred by him to his typfcal species. One of his skulls, however, 

 proves to be a part of the pectoral girdle of his original type, an error 

 resulting from the confusion of the interclavicular foramen with 

 the pineal foramen, an error not so great as it would seem, since such 

 a foramen in the pectoral girdle was unknown previously among 

 plesiosaurs. A close comparison of the two skulls, both from the 

 same horizon and adjacent locahties, leaves no doubt of their identity, 

 both generically and speciiically. Skull " No. 2, " the better specimen, 

 shows the underside nearly complete, with the hyoids and jaws in 

 place, a small part of the facial portion only, wanting. The upper 

 part of the skull is so badly compressed and mutilated that but little 

 decisive can be made out from it, but the palatal structure, perhaps 

 the most important of the' plesiosaur cranial anatomy, has been 

 determined from this specimen almost perfectly, the relations of the 

 vomers only being doubtful (figs. 1,2). 



The slender parasphenoid separates two long and narrow openings 

 between the pterygoids, which openings may properly be called the 

 parasphenoidal vacuities for the present. On either side, the lo'ng, 

 broad, gently concave plate of the pterygoid fills up nearly all the 

 free space between the vacuity and the mandible, and thence extends 

 forward as a slender process on either side of the real interpterygoidal 

 vacuity to articulate externally with the palatine, and anteriorly, 

 doubtless as in T. osborni, with the vomer. The true interpterygoidal 

 vacuity is an elongated ovate opening, obtuse posteriorly, acute in 

 front, bounded by thickened, rounded margins; posteriorly it is bor- 

 dered by the slightly expanded anterior end of the parasphenoid, which 

 is here concave in outline and thick, the pterygoid suture extending 

 forward on each side from the front end of each parasphenoidal 

 vacuity. In the type specimen of T. osborni the anterior end of this 

 bone was somewhat mutilated, and I was not quite certain that it was 

 not produced forward to fill up this vacuity as in other known plesio- 

 saurs. This, however, is not the case. This extraordinary opening 

 is therefore unique for this genus and Polycotylus among plesiosaurs, 

 and indeed among all known reptiles, if it be merely a vacuity. And 

 I venture the opinion that it may be compared with a similar opening 



