272 Rev. L. Jenyns on some of 



dark-coloured variety of the A. arvalis noticed under the head 

 of that species. 



With a view to inquire still further into the characters of 

 the A. neglect a, by permission of Mr. Thompson I dissected 

 several specimens, and compared their internal structure with 

 that of the A, arvalis ; but, excepting in the cranium to be 

 hereafter noticed, no very obvious differences presented them- 

 selves. There are a few points, however, in relation to this 

 subject, which may be worth stating. 



The length of the intestinal canal, as well as the relative 

 length of its different portions, both in the A. neglect a and 

 the A. arvalis, varies much in different individuals, and even 

 in individuals of the same size and sex. Mr. Yarrell* and 

 Mr. MacGillivrayt have both given measurements of these 

 parts in the A. arvalis, which are very different from each 

 other, but which, as the latter gentleman has not mentioned 

 the size of the individual from which they were taken, do not 

 admit of direct comparison. I shall annex the results which 

 I obtained in three different instances of the A, neglecta, and 

 one of the A, arvalis. 



No. 



in. 

 Small intestines ... 13 



CsBcum 6 



Large intestines ... 12 



No. 1. was a male neglect a, measuring four inches in length, 

 exclusively of the tail. No 2. was a female of the same spe- 

 cies, and exactly of the same size. No. 3. was a young male 

 of the same species, measuring three inches. No. 4. was a 

 male arvalis, exactly of the same size as No. 3. It will be ob- 

 served, that Nos. 3. and 4, which are different species, do not 

 differ more in this respect than Nos. 1. and 2, which are sexes 

 of the same. 



Another part w^hich I found varying in different individuals 

 was the gall-bladder. It is observed in anatomical works that 

 this organ is found wanting in many of the Roderitia, parti- 

 cularly ^among the Rats J. Mr. Yarrell observes, that both the 

 field and bank Campagnol are equally devoid of it. If it be 

 really the fact, that it is never present in the former of these 

 two species, this circumstance will tend to the confirmation 

 of the A, neglecta being distinct, in which I have observed it 

 in the only three cases I have examined, though of such dif- 

 ferent degree of development as to lead to the suspicion that 



* Lond. Mag. Nat. Hist, vol. v. p. 600. f Brit. Quad., p. 267. 



X Blnm. Man. Comp. Anat., by Lawr. (2nd edit.), p. 128. 



