NORTH AMERICAN PLESIOSAURS 719 



united pterygoids clearly intercalated between them. Between the 

 pterygoids and parasphenoid, and hollowed from the latter there ap- 

 pear to be two parallel canals, terminating on the free surface below. 

 Along the middle of the rounded under surface of the parasphenoid 

 there is seen a slender groove, as though the remains of a suture, a 

 remarkable thing if it be really a suture. The parasphenoidal 

 vacuities are bounded outwardly by the everted margins of the 

 pterygoids, internally by the parasphenoid. The openings, however, 

 do not quite reach to the hind end of the external boundaries, the 

 pterygoids forming a floor to the grooves on the posterior fourth, as 

 seen from below. So extraordinary a development of the para- 

 sphenoid in this group of reptiles as is shown in these specimens is of 

 more than passing interest. If it be the true vomer of the mammals, 

 one cannot understand the cause of its retention in so highly devel- 

 oped a condition unless the plesiosaurs sprang from reptiles that had 

 not yet lost it. The Nothosauria are markedly different in the com- 

 plete union of the pterygoids on the median line, to the exclusion, not 

 only of the parasphenoid but the basisphenoid also; a condition, 

 which, associated with the typical reptilian phalangeal formula, 

 excludes the group I believe absolutely from direct and perhaps 

 indirect genetic relationships with the plesiosaurs. Furthermore, the 

 persistent retention of the parasphenoidal vacuities, so definitely 

 bounded in all plesiosaurs, is puzzling, unless the explanation is that 

 suggested by me in my earlier paper — that they are the real nareal 

 openings. 



The crushed condition of the upper part of the skull is such that 

 little definite can be made out, though the resemblance to the same 

 parts in T. osborni is evident. On the left side the jugal arch has been 

 but httle disturbed, showing the jugo-postorbito-squamosal sutures 

 as in T. osborni. There is no indication whatever, and the parts 

 are here intact, of a quadratojugal bone, the supposed suture of 

 T. osborni being in no wise apparent. I believe that at last it may be 

 definitely said that the quadratojugal bone is absent in all plesiosaurs, 

 as a distinct element. 



Unfortunately the jumbled condition of the frontal and prefrontal 

 elements in these skulls has obliterated all sutures. An examination, 

 however, of the various plesiosaur skulls in the British Museum and 



