68 MR. S. J. A. SALTER ON THE 
distinctive differences are considerable and many: some of the 
principal I will enumerate. By reference to the accompanying 
figures they will be easily recognized; and the drawings have 
been rendered twice life-size to make the distinctions more con- 
spicuous. The proportions have been retained with scrupulous 
care*. 
Commencing with a top view of the skull, the nasal bone is seen 
in Mus rattus to be broad and obtuse at its anterior extremity, 
bulging out somewhat suddenly ; whereas in the other skull it is 
more pointed, and it increases from behind forwards by an even 
line. In Mus rattus the infra-orbital foramina are nearly twice as 
large as those in the other skull, while the interval between these 
foramina is barely more than half, showing a much larger nasal 
capacity in the new rat. In the latter the zygomatic arches are 
nearly straight; in the former they are much bowed. The fronto- 
parietal suture is crescentic in the new rat; it is nearly straight 
in Mus rattus. In the former there is a strongly marked cres- 
centic ridge for muscular attachment passing across the parietal 
bones ; this is totally wanting in the latter. The lambdoidal suture 
in the new rat is truly lambdoidal; in the other it is an irregular 
wavy line passing across the skull. In this view of the cranium 
the molar teeth are visible in Mus rattus, whereas they are hidden 
in the other skull. 
But the most important and weighty distinction between the 
two skulls is the size and form of the foramen magnum occipitale, 
as seen on the posterior view of the cranium. In the new rat the 
foramen is nearly circular, with two small lateral notches, and 
comparatively small ; in the old Black Rat it is oval, with a central 
curved notch above, broad from side to side, and very large. 
Fig. 3. Fig. 4. 

Posterior view of Cranium of Posterior view of Cranium of 
the Snake-Rat. Enlarged Mus rattus. Enlarged two 
two diameters. diameters. 
* T am indebted to my brother, Dr. Hyde Salter, F.R.S., for these accurate 
drawings. 
