480 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



Where they disappear beneath the premaxillge they have a combined 

 width of about 35 mm. On each side the parietal turns downward 

 and sHghtly outward into a thin descending process or wing, forming 

 the lateral wall of the brain case to a depth of 60 mm. At the 

 anterior inner angle of the temporal vacuity there is a rather strong 

 emargination of this descending wall for the attachment of the epip- 

 terygoid, from the upper margin of which a somewhat zigzag sutural 

 line runs upward and then forward to join the extremity of the 

 suture between the prefrontals and frontals. These lines appear to 

 be quite alike on the two sides and since they agree with the sutural 

 divisions in Trinacromerum oshorni and also with the recognized 

 sutures in the skull described by Andrews as Pliosaurus ferox, there 

 can be no question, I think, but that they indicate the divisions 

 between the parietals and postfrontals. Back of the parietal fora- 

 men the parietals show no clear indications of a median suture. The 

 part here, for the rather long distance between the temporal vacuities, 

 is obtusely rounded above and nearly horizontal, very unlike the thin, 

 elevated crest of Trinacromerum, Polycotylus, and Elasmosaurus. 

 On the under side the parietals include a deep valley between the 

 lateral wails, a little wider below and meeting in a rounded roof 

 above, for the brain case. This cavity measures over 50 mm. in its 

 greatest width. 



The arrangement of the bones in the frontal and antorbital regions, 

 as I have described them, whatever may be their interpretation, 

 doubtless obtains in all plesiosaurs, with minor modifications. 

 Wliichsoever interpretation may be finally accepted the arrange- 

 ment and structure are very remarkable and very unlike what is 

 known in other reptiles. That the bones are nearly or quite as I have 

 described them in this specimen I have no doubt. Andrews, in his 

 figures and description of the skull of Pliosaurus ferox, reaches differ- 

 ent conclusions and has different interpretations, but I am confident 

 that, if his specimen be studied in the light of the information furnished 

 by the present one, other conclusions and other interpretations will 

 be reached. A positive suture has never been detected separating 

 the median bones in front of the pineal foramen from the parietals. 

 Owen, it is true, thought he detected such a suture in a species of 

 Plesiosaurus, and Andrews thought there was one in his Pliosaurus 

 ferox specimen, though he adds that the parietals are probably 

 anchylosed with the ''frontals." I have been unable to distinguish 

 such a suture in four well-preserved skulls of as many different 

 genera studied by myself. In the specimen of Trinacromerum oshorni 

 studied by me, while the adjacent sutures are all clearly indicated, 

 save such as were obliterated by crushing, the very narrow prolonga- 

 tions in front of the foramen have no trace whatever of a distin- 

 guisliing suture, either on the upper or the under side. I believe them 



