312 MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 



mandible at this part rises to an obtuse angle, and is here 1^ inch high, or deep, 

 whilst its thickness here is more than one-third of an inch, lessening to one-eighth 

 towards the smooth rounded inferior margin. A row of vascular holes lies a little below 

 the upper margin all along the dentary element ; and the whole of the part which is 

 covered with strong horn is grooved and rough, especially towards the upper part. A 

 rough thick margin also passes within the ramus below ; whilst the inner surface, 

 although richly dendritic with vascular grooves, is smoother than the outer. Three- 

 fifths of the lower margin of the jaw is rough and covered with horn ; the thickened 

 tubercular structure then passes upwards, almost parallel with the upper margin of the 

 jaw, but converging slowly to meet and blend into that margin at the most elevated 

 part of the mandible. There is, towards the end of the symphysial line above, a trian- 

 gular fossa, passing on each side into an inner submarginal groove, which groove 

 slowly becomes obsolete in the posterior inner part of the dentary'. All that posterior 

 part of the mandible which is not covered with horn is very smooth and polished, both 

 without and within. The thickness of the dentary part of the mandible is pretty 

 uniform, averaging about five lines — the thickest or widest part being at the symphysis. 

 The height is very uniform for the first three inches, being about half an inch ; it then 

 gradually increases, until at the zygoma it is one inch and a third. The upper edge of 

 the mandible at first rises for one-third of an inch ; it then forms one gentle concave 

 sweep to nearly the highest point ; the ideal basal line to this arc being 5f inches 

 in extent, and the mid-portion of the arc three-fifths of an inch from this basal line. 

 From the gradual rise of the upper margin, and the increasing depth of the jaw behind, 

 the outline of the lower margin is not so convex as the upper is concave. 



The extent of the symphysis is 1^ inch ; it is grooved below, and above has at first 

 a convexity, then a slight keel, whilst the latter third is occupied with the triangular 

 fossa above mentioned. Behind the highest part on the upper margin the bone 

 descends, at first slowly, then rapidly, to the external articular facet, which, more than 

 three-fourths of an inch in extent, passes backwards and inwards. The widest part of 

 this facet is the middle, which is also the most convex ; the narrowest and most con- 

 cave part is the inner. Anterior and interior to this is the deep double articular sulcus 

 for the anterior condyles of the quadratum ; the deepest and longest of these twin sulci 

 is the external. They are together nearly half an inch across, and their anterior margin 

 1^ inch from the posterior edge of the outer articular surfaces. These exquisitely- 

 cut and adapted surfaces answer, as will be seen at once, to the lower condyles of the 

 OS quadratum. 



The internal angular process projects inwards and backwards half an inch from the 

 innermost articular groove ; a ridge passes across and connects it with the posterior 

 end of that groove, and behind this ridge there is a large oval pneumatic foramen 



' This fossa, and these diTerging grooves are imprints of a very important embryological structure ; they show 

 where the cartilages of Meckel ran and met each other (infra, p. 315). 



