OF THE BEAVER AND OF SOME SQUIRRELS. 1211 



glands is also discharged. These are represented by a pair of 

 immense sacs, with puckered walls, one lying on each side. 

 They are filled with fluid ; and the aperture of each sac opens 

 close to the median aperture of the penis, the three apertures 

 lying close together just within the common orifice above 

 mentioned. (Text-fig. 60, A, C, E.) 



The arrangement is very similar in the female, in which the 

 anal and castorium glands appear to be as well developed as in 

 the male. The chief difierence is that the orifice of the vagina 

 opens separately into the cloaca. Just in front of it there is a 

 transverse slit in the wall of the cloaca, and when this is opened 

 the apertures of the castorium glands may be seen on each side 

 of it, just in front of that of the vagina. (Text-fig. 60, B, D.) 



To sum up, the Castoridje difi^er from the Sciuridse, so far as 

 the organs just described are concerned, in the following parti- 

 culars : — (1) In both sexes the anus and genitalia open into a 

 common chamber or cloaca with a single external orifice. 

 (2) The anal glands are of great size and open apart from the 

 anus. (3) There is a pair of large castorium glands opening 

 close to the generative orifice. (4) In the male there is no 

 scrotum; the penis is straight and is protrvided close to the 

 anus. 



In view of these well-known and profound difierences, it is 

 singular that in the current text-books the Castoridfe are 

 separated from the Sciuridee merely by such comparatively 

 trivial difli'erences as are presented by the skull and teeth, the 

 webbed hind feet and broad tail being added as of secondary 

 value, and no mention whatever being made of the genitalia 

 and anus. The difference between the Beavers and Squirrels 

 in the anus and external genitalia may be summarized as 

 follows * : — 



a. Sexes externally alike owing to the anus and genito-urinary orifices 

 opening into a cloaca with a single aperture at the root of the 

 tail, no hairy prepuce or vulva, the concealed orifice of the 

 penis just in front of the anus; testes inguinal; a voluminous 

 " castorium " gland opening close to the orifice of the penis and 

 vulva ; anal glands very large, with long tubular orifice Castoridce, 



h. Sexes externally unlike, the anus and external genitalia not 

 opening into a cloaca ; female with hairy vulva close beneath 

 anus ; male with long perineal region, typically with large 

 scrotum for testes in breeding season ; penis long, opening by 

 prominent hairy prepuce on posterior part of abdomen ; no 

 " castorium " gland ; anal glands small, with small orifice. 



Sciuridce, Petauristida. 



From the above-given analyses of the external characters of 

 the Castoridte, Sciuridse, and Petauristidae it will be seen that, so 

 far as the organs considered are concerned, there is not a particle 

 of evidence of kinship between the OastoridaB and the other two 

 families. 



* I have not in this paper dealt with the great differences between the Beavers 

 and Squirrels in the tail, because they are well known. 



