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 hone 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 



t lie Snake- Put. Enlarged Mus r alius. Enlarged two 



two diameters. diameter*. 



* T am indebted to my brother, Dr. Hyde Salter, F.R.S., for these aceurate 

 drawings. 



