398 



face, the two surfaces being defined by the temporal ridge, which extends horizontally for- 

 wards to bound in like manner the upper surface of the frontals. The temporal fossa is 

 bounded behind by a vertical ridge, some way in advance of the sides of the superoccipital 

 ridge. There is a crescentic vacuity between the squamosal and tympanic. The antorbital 

 vacuity is defended externally by a subvertical plate upon the front part of the maxillary root 

 of the zygoma, and there is a direct opening into the nasal chamber anterior to the orbit 

 besides the vertical curved, probably lacrymal, canal, external to the alveolus of the upper 



incisor. 



Presented by William Clift, Esq., F.B.8. 



2228. The skull of a Norway Rat (Mus decumanus}. The rami of the jaws are sepa- 

 rated, and the teeth exposed in situ. Presented by William Clift, Esq., F.R.S. 



/ 



2229. The skull of a Norway Rat (Mus decumanus). Presented by Henry Cline, Esq. 



2230. The skull of a large Norway Rat (Mus decumanus), longitudinally bisected. 



The petrosal has a deep cerebellar fossa : the cochlear and vestibular orifices commence 

 distinctly upon its inner surface, and not from a common ' meatus internus.' A prominent 

 acute-angled ridge divides the anterior from the posterior surface of the petrosal. The rhin- 

 encephalic chamber is of proportionally large size. There is no sella, or pituitary depression, 

 nor clinoid processes. The bony canal, which commences at the fore part of the antorbital 

 vacuity, and bends round the outside of the incisive alveolus, inclines forwards, and termi- 

 nates below the attachment of the anterior turbinal to the premaxillary. 



Presented by Prof. Owen, F.R.S. 



2231. A transversely bisected skull of the Norway Rat (Mus decumanus}. 



Presented by Prof. Owen, F.R.S. 



2232. The mutilated skull of a large male Norway Rat (Mus decumanus}, with un- 

 usual elongation of the incisors, the consequence of want of mutual appo- 

 sition. Presented by Dr. Leach, F.L.S. 



2233. The skull of a female Norway Rat, showing a similar abnormal growth of the 

 incisors from the same cause. Presented by H. Carwardine, Esq. 



2234. The skull of a Norway Rat (Mus decumanus). 



The teeth removed from the left side of both upper and lower jaws, and separately dis- 

 played. The grinders have diverging roots or fangs, as in other omnivorous Rodents. The 

 large premolar in the upper jaw has not fewer than five fangs. 



Hunterian. 



