| BARYONYX WALKERI 
Fig. 12 Baryonyx walkeri, holotype, BMNH R9951; partial reconstruction of lower jaw, in medial (lingual) aspect. x 0.135. 
are much narrower than are the margins of the stout anterior part of 
the dentary, but they are still somewhat thickened anteriorly, being 
backward continuations of the lips of the ever-widening Meckelian 
groove and continuing to curl round towards each other as distinct 
overhangs. The dorsal overhang forms a slot for the surangular. As 
‘the blade widens posteriorly its ventral lip, which forms a slot for the 
angular, diminishes and eventually disappears altogether; the form 
of this fragile posterior end of the dentary is indicated only by an 
outline that has, in places, been badly damaged and eroded. The 
ventral margin is nevertheless preserved intact and forms a smooth, 
slightly convex curve. Its extreme tip (as preserved) is probably not 
far short of what was once the posterior end of the complete dentary. 
From that end the dorsal margin of the dentary ascended 
anterodorsally to the widest point and then descended again for- 
‘wards, but this part is extremely thin and, as preserved, has an 
irregularly broken edge. However, at two places on this broken edge 
there are small lengths of smooth-edged bone of a curved outline, 
recessed from the general level of the surface, which are doubtless 
arts of the margins of dentary fenestrae. 
In dorsal view the lingual margin of the dentary forms a smooth, 
ently concave curve, somewhat more marked in front of the 9" 
alveolus than behind it. The labial margin, by contrast, curves 
outwards markedly to accommodate the roots of the greatly enlarged 
‘rst 5 teeth; this swelling produces the mandibular portion of the 
inal rosette’. 
_ The Meckelian groove runs lengthwise along the lingual surface 
of the fragment, at a little below mid-height. It has a strongly 
overhanging dorsal lip and a fairly well-marked ventral lip, the gap 
detween them being 16 mm; the base of the groove is a smooth 
as Anteriorly the groove becomes shallower and narrower, its 
ips become less pronounced, and it peters out anteriorly some 60 
nm behind the symphysis and beneath the wall separating the 4" and 
t alveoli. Posteriorly, from the 19" alveolus backwards, the lips of 
he Meckelian groove are themselves grooved and facetted for 
irticulation with the splenial. Indeed, the major part of the lingual 
urface of the posterior end of the dentary, tapering forwards into the 
eckelian groove, served as a slot for the splenial. 
The labial surface of the dentary bears a number of foramina, 
_vhich doubtless served for the passage of blood vessels and/or 
erves. There are about 25 scattered over the expanded area at the 
nterior end, with a much smaller number farther back. There is also 
linear series of mental foramina running parallel to the dorsal 
dargin of the bone, lying in a shallow groove at approximately the 
evel of the bases of the alveoli. These do not, however, bear a fixed 
‘umerical relationship to the alveoli, being significantly fewer than 
pe latter. They served for the passage of branches of the inferior 
Iveolar nerve and also, presumably, blood vessels. The last foramen 
f this linear series happens to be on the anterior end of the posterior 
nen behind this the shallow groove continues all the way to the 
osterior end of the dentary as preserved. 
The dorsal border of the lingual surface of the dentigerous region 
of the dentary is, for most of its length, of approximately the same 
height as the dorsal border of the labial wall and as the interdental 
plates. The interdental plates are, therefore, barely exposed in 
lingual view, unlike the usual theropod condition (e.g. Allosaurus). 
The alveoli themselves vary in form, from squarish/circular in 
occlusal view (2") to egg-shaped (3") to a rectangle twice as broad 
as long (9"). They vary also in size (see ‘Dentition’). They are 
immediately adjacent to each other so that they are separated by no 
more than a thin interalveolar wall. The lingual wall of the row of 
alveoli, i.e. the row of interdental plates, is separated from the dorsal 
rim of the lingual surface of the dentary by a deep paradental groove 
(the ‘nutrient groove’ of Osborn 1912). However, each alveolus 
communicates with the nutrient groove by means of a slot in its 
interdental plate, with a foramen at its base. For most of the tooth 
row these slots are narrow and situated posterolingually (see Fig. 
14), but, in the case of the larger anterior teeth nos. 1-5, they are 
much wider and more anterior in position. 
SPLENIAL (Fig. 15). The right splenial is preserved complete save 
for the fragile dorsal margin and posterior end. The left splenial, by 
contrast, is very incomplete, the only part of its true margin remain- 
ing being a small piece of its posterodorsal corner. 
The splenial is a thin, narrow, subtriangular sheet of bone, its apex 
directed anteriorly, which was slotted into the Meckelian groove on 
the lingual surface of the dentary. The medial (or lingual) surface of 
the splenial is distinctly convex dorsoventrally, the convexity being 
more marked in the posterior half of the bone. The lateral (or labial) 
surface, facing the dentary, is correspondingly concave. Midway 
along the length of the element, just above its ventral border, is the 
moderately large splenial fenestra (= the anterior mylohyoid foramen 
of Currie & Zhao 1993 and the splenial foramen of Madsen 1976, the 
latter labelled Meckelian canal in his Plate 9), elongated in an 
anteroposterior direction. This fenestra is completely surrounded by 
bone in Baryonyx and Sinraptor but is in a marginal position in 
Allosaurus; in Ceratosaurus it is only small, and in Coelophysis and 
Dilophosaurus it is absent altogether. 
The medial surface bears a sharp ridge just above its ventral 
margin, which, by virtue of its overhanging nature, produces a 
ventral-facing groove that received the dorsally directed, wrapped- 
around lower margin of the Meckelian groove on the dentary. 
Anterior to the splenial fenestra this splenial groove is well devel- 
oped; beneath the splenial fenestra it is very shallow, forming 
nothing more than a faint lip; behind the fenestra the groove is again 
distinctly present, but it is narrower and shallower than in the 
anterior region and posteriorly it fades out altogether into a flat, 
narrow, ventral margin to the bone. 
The dorsal margin of the splenial rises to a ‘dorsal process’. 
Anterior to that the edge of the bone, although a little crushed, is 
almost straight. Posterior to the dorsal process, the dorsal margin of 
