N... 1540. THE SKULL OF BKACHATT'HENLUS—WLLLTSTOX. 481 



to be merely exogenous processes from the parietals, produced for- 

 ward to meet the extraordinarily elongated premaxillary processes. 



If such be really the case, the bones on their outer sides must of 

 course be the frontals, and, as frontals, they occupy their normal rela- 

 tions with the adjacent bones, save only the parietals, articulating 

 behind with the postfrontals, in front exteriorly \vdth the prefrontals, 

 anteriorly with the maxillae and premaxillae. If the median bones be 

 really the anchylosed frontals, then these bones must be the nasals. 

 As such, however, their relations would be most extraordinary, the 

 only instance in comparative osteology where they articulate with the 

 postfrontals and postorbitals. 



Possibly the same causes which have prolonged so far backward the 

 premaxillaries may have caused a posterior displacement of the 

 nasals. In any event I feel sure that the bones on the outer sides of 

 these, the supposed supraorbitals, the ones bordering the orbits and 

 reaching to the nares, are the real prefrontals. As such their position 

 and relations are not extraordinary. As supraorbitals they are quite 

 indefensible. 



If the former interpretation be correct, that the parietals have 

 excluded the frontals from contact in the middle line, the nasals are 

 wanting in the plesiosaurs. If the latter interpretation is correct, 

 then all the elements of the normal reptilian skull are present, but the 

 nasals have become abnormal in position and relations. I do not 

 know how the problem can be settled, unless, indeed, some favorably 

 preserved specimen may disclose an actual suture in front of the 

 parietal foramen. 



Lachrymals. — The lachr^anal is an elongate bone forming the lower 

 anterior half of the orbital margin. Its sutural union with the pre- 

 frontal is very evident on each side; the suture between it and the 

 maxilla is perhaps not wholly free from doubt in this specimen, 

 though there can be little possibility of error, the indications of the 

 two sides agreeing as they do. The bone joins the jugal behind by an 

 oblique suture; the maxilla in the middle below; and the prefrontal 

 anteriorly, as already described. Inasmuch as these relations seem 

 to be quite the same as those described by Andrews in Pliosaurus 

 ferox, I think that the presence of a lachrymal as a distinct bone in the 

 plesiosaurs may be finally set at rest. In the skull of Trinacromerum 

 oshorni, previously described, there is a pointed process of bone which 

 has the same relations with prefrontals and maxilla?, but not with the 

 jugal. I could not detect a suture separating it from the maxilla. 

 Neither is it probable that the lachrymal in Elasmosaurus snowi, 

 which must resemble that of Trinacromerum, articulates with the 

 jugal. 



Postfrontals and postoriitals. — The postfrontals and postorbitals I 

 believe are distinct bones in this specimen. The parieto-post- 



