“- 
THE BLACK OR SHIP RAT sol 
of a parietal, measured along a crest, is considerably less than, instead 
of about equai to, the greatest distance between the crests. The 
parietals are vaulted noticeably above the general dorsal profile, a 
feature emphasised by a slight flattening of the frontal and inter- 
parietal regions. The large inter-parietal extends relatively far back- 
wards, and its boldly convex posterior border produces a characteristic 
backward deflection (from the plane of the mastoid processes) of the 
central part of the lambdoidal crest; as a consequence the occiput is 
vertical, the condyles are completely hidden in the dorsal view, and 
the occipito-nasal length amounts to from 105 to 106-5, instead of 102 
to 104:5 per cent. of the condylo-basal length. The rostrum is relatively 
slender; its width amounting to from 17-6 to 20-3, instead of 19-5 
to 22-3 per cent. of the condylo-basal length ; the blade-like outgrowths 
of the premaxillae beneath the nasals are rather large; the zygomatic 
arches as a whole are more slender; and the palatal length and the 
diastemata (in Z. 7. rattus, alexandrinus and frugivorus) are relatively a 
little shorter than in the Brown Rat. The auditory bulle are relatively 
a little larger and more inflated than in worvegicus, and they com- 
press to some extent the anterior part of the basi-occipital, the width 
of the latter bone in front being rather less instead of slightly more 
than half its median length. 
As in all other mammals, growth produces many changes in the 
proportions of the skull, the facial region being the chief seat of post- 
natal growth. The following measurements from Hossack illustrate 
the process in the present species :— 







| 
“ Tnter- Length of Upper 
casa orbital zygomatic | Diastema. molar Tangttot 
| = 3 breadth. plate. series. Z 
Extreme length. 
Percentages. 
| 
Young, 28 mm.=100 A C 50 19°6 8-9 25 25 33°9 
Adult, 41°5mm.=100 . - 37-4 13°9 124 275 17-1 35°7 




Apart from its smaller size the mandible does not differ importantly 
from that of zorvegzcus. 
The cheek-teeth (Plate XX VIII., Fig. 8) have the outer cusps slightly 
less reduced, and in the upper teeth, the median tubercles slightly less 
developed than in xorvegicus. In m cusp 1 is distinct and usually 
nearly as large as 7; as a rule there is no trace at all of an anterior 
cingulum. In 72 cusp 5 is still evident although partially fused with 
&; in one case Forsyth Major saw this cusp placed so far forwards that 
it blocked up the entrance to the transverse sulcus. In 7 a minute 
trace of cusp I may be present occasionally, but 5 is completely lost. 
In m, cusp 6 is apparently a constant feature, but 2 is always more 
