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Shales at Kettleness on the north Yorkshire coast (Benton & Taylor 
1984) of Toarcian (Bifrons Zone) age. Other English species that 
have been referred to this genus include Plesiosaurus megacephalus 
Stutchbury, 1846 and P. propinquus Phillips, 1854. Only R. 
megacephalus is represented so far by more than one specimen, and 
it alone seems to be from the Lower Liassic (Rhaetian/Hettangian) 
(Cruickshank 1994a). Plesiosaurus propinquus differs from other 
species in having a marked boss on the hind end of the inner surface 
of the lower jaw, just in front of the glenoid and in place of the dorso- 
median trough (Taylor 1992a, b), and thus its position within 
Rhomaleosaurus must be reconsidered. 
Table 1 Abbreviations used on Figs 1-6. 
alv anterior interpterygoid lgr lateral groove 
vacuity mto mature tooth 
carina carina on crown of tooth mx maxilla 
co coronoid no notch 
cr crown orb orbit 
d dentary pal palatine 
dep depression palv primary alveolus 
dmfo dorsomedian foramen pmx premaxilla 
ec ectopterygoid po postorbital 
en external naris pt pterygoid 
fac facial processes of the ptb pterygoid boss 
premaxillae ri ridged ornament on crown 
fan fan-shaped area of tooth 
fo foramina Tto replacement tooth 
er groove salyv secondary alveolus 
in internal naris sof suborbital fenestra 
info foramina associated with sp splenial 
internal naris sym symphysis 
j jugal Vv vomer 
1 position of first tooth 
Oblique lining represents broken or sectioned bone or tooth. 
Mechanical stipple represents matrix or crushed bone. 
DESCRIPTION. Skull (Figs 1, 2). The skull and lower jaw have 
recently been cleaned and conserved, and they alone will be dealt 
with here. Only the anterior portion of the skull was collected; the 
clean break surface runs obliquely from a position in front of the left 
orbit, through the left external naris, to the front edge of the right 
orbit, and thence through the postorbital bar. Some bone has been 
lost from the tip of the premaxillae. The right cheek bar is attached 
to the snout and runs as far as the end of the maxilla. Attached to the 
cheek bar is a portion of the palate, comprising the right ectopterygoid 
and a small part of the pterygoid. The base of the postorbital rests on 
the posterior end of the jugal. Apart from the obvious break, the skull 
has been damaged by post-mortem effects which have compressed 
the bone dorso-ventrally and caused the facial processes of the 
premaxillae to be shortened, so that the midline of the snout has a 
step, with the posterior part of the premaxillae, as preserved, being 
pushed under the anterior part and offset to the right. The maxillae 
may, in addition, have been squeezed together under the facial 
processes of the premaxillae. All this disruption has obscured the 
right external naris. In front of, and lateral to, the position of the 
hidden right external naris, is a deep depression bottomed with 
crushed bone and an associated wide groove running to the premax- 
illary edge. The right jugal is partly visible, and is a narrow bone 
running under the orbit and ending below the postorbital. However, 
as the bone is heavily pyritized and crushed; and the sutures much 
closed up, the prefrontal and lacrimal cannot be distinguished. 
Similarly the detailed structure of the postorbital — jugal area is 
obscured. There is no reason to believe that this latter region is any 
different from that described in R. megacephalus (Cruickshank 
A.R.I. CRUICKSHANK : 
dmfo 
See i ea 
RL Ta 
dep 
RS 
ptb 
Fig. 1 Rhomaleosaurus thorntoni Andrews; dorsal view of the skull; 
scale bar = 100mm. For abbreviations on this and the other figures, see 
Table 1. 
1994a), or indeed the Kimmeridgian species Pliosaurus brachy- 
spondylus (Taylor & Cruickshank 1993). 
The anterior palatal surface shows much less damage. A very few 
fully erupted (mature) teeth are still in their sockets, but several 
replacement teeth are present, both in primary and secondary al-| 
be distinguished except the premaxillae and maxillae. The vomers 
are substantial bones, forming a midline bar on the palate. Anteriorly/) 
they terminate in a horseshoe-shaped structure with several associ] 
ated foramina. The vomers widen posteriorly, and are here flanked 
by grooves which run to the internal nares from fan-shaped areas just | 
behind, and internal to, each diastema, opposite the notches where} 
the premaxillae meet the maxillae. These fan-shaped areas arel] » 
= 
