Hatcher: Litile Known Fossii, Vertebrates. 137 



TiiK Mandible. 



The i^rescnt description of the mandible of Trii:;onias osbonii is 

 based on a complete lower jaw (No. 97) of a somewhat young ani- 

 mal, in which M.^ is seen in process of eruption. The jaw is pro- 

 portionately long and remarkably slender. It does not appreciably in- 

 crease in depth from the posterior border of the symi)hysis to the 

 posterior border of M..t. The inferior border is gently convex. 

 There is a mental foramen directly below the anterior root of V.^. 

 The angle is not [iroduced !)elow the inferior border of the jaw. The 

 angle is slightly deflected and there is a short, gently concave area 

 between its anterior extremity and the preceding inferior border of 

 the jaw. The condyle is placed almost at right angles to the ascend- 

 ing ramus, with the articular surface somewhat more elevated and 

 broader externally than internally. The coronoid process is well de- 

 veloped and pointed posteriorly at the apex. The masseteric fossa is 

 deep and situated rather high up on the ascending ramus ; its inferior 

 border is on a line approximating that of the crowns of the inferior 

 teeth. The inferior dental foramen is placed somewhat nearer the in- 

 ferior than the superior border and just behind the last molar. The 

 symphysis is long, narrow, and obliquely horizontal. It terminates 

 posteriorly immediately below the mental foramen, and there is a de- 

 cided constriction anterior to P.y. 



The Dentition. 



The dental formula is I.f, C.^, P.i, M.f = 42. 



The superior dciilition : I.i is hypertrophied, and is a large, laterally 

 flattened tooth, with a chisel-shaped, rectangular crown. It is much 

 the more persistent of the superior incisor series. Incisors^ and ^ are 

 lost in both of our skulls, but the alveoli are preserved on either side 

 in No. 96, and the same is true of the superior canine, which is sepa- 

 rated from I.^'- by a short diastema. Between the canine and P.i there 

 is a long diastema. P.i is unfortunately represented in both our 

 crania only by the roots. These indicate that this tooth was rather 

 large and well fixed in the jaw for a su[)erior first premolar. The suc- 

 ceeding teeth are all present in both our skulls, but too much worn in 

 No. 96 to permit of a description of their structure, except in M.-3-, 

 which is fairly well preserved. In No. 98 these teeth are well pre- 

 served and show the following characters : The premolar series increase 

 in size anteroposteriorly. None of the superior premolars are molari- 



