the smaller British Mammalia, 273 



it may sometimes be wanting here also. In one individual it 

 was of considerable size, attaining to the margin of the liver; 

 in a second it was less ; in a third it was very small, but still 

 obvious. I have not observed it, any more than Mr. Yarrell, 

 in the true A. arvalis. 



The A. neglecta and the A. arvalis agree in the number of 

 vertebrae. M. de Selys-Longchamps has given the entire 

 number in the latter as forty-six, which accords with the num- 

 ber given by Mr. Yarrell, as well as with that observed by 

 myself in several individuals of each of the above species, un- 

 less a very minute rudimentary one at the extremity of the 

 tail be included, in which case the entire number must be set 

 at forty-seven. In one instance, however, of the A. neglecta, 

 a female, I found an additional caudal vertebra, making the 

 entire number forty-seven without the rudimentary one. This 

 affords another proof of the caution that is required in draw- 

 ing any conclusion as to the number of vertebrae from the ex- 

 amination of single specimens. 



The number of ribs was in all cases the same for both spe- 

 cies, seven true and six false ; being also the number given to 

 the A. arvalis by De Selys. 



The only part of importance remaining to be mentioned is 

 the cranium. I have examined that of three individuals of the 

 A, neglecta, and, though in general form the same, I find it 

 decidedly larger, broader across the zygomatic arches, and 

 with the bones of the zygoma itself stronger, than that of the 

 arvalis, comparing two individuals of the same size in other 

 respects. In the adult animal, the strength and bend of the 

 zygomatic bones become very considerable, indicating great 

 niuscular powers in biting and masticating its food. The fol- 

 lowing are the relative measurements of the crania of a large 

 and small A. neglecta, and also of that of the A, arvalis for 

 comparison. 



No. 1. No. 2. No. 3. 



in. lin. lin. lin. 



Entire length 1 11 10| 



Breadth across zyg. arches 7 barely. 6 exceeding. 5| 



Breadth behind the zyg. arches 5^ 4| 4| 



No. 1. is that of an individual of the A. neglecta, measuring 

 four inches in length, tail excluded. No. 2. is that of another 

 individual of the same species, measuring three inches. No. 3. 

 is that of an individual of the A, arvalis, exactly of the same 

 size as the last. 



Having entered above, in some detail, into the characters 

 of the A. neglecta, and the grounds on which I venture to 

 confirm Mr. Thompson's opinion, as to its being distinct from 



Ann. ^ Mag. N. Hist. Vol. vii. T 



