146 Mr. 0. Thomas on 



thoracic width. Male sexual orifice situated above the 

 interval between the legs of the first and second pairs. 



Note. — In one male specimen the tracheal tubes leading 

 from the capitulum are quite distinct; each is at first double, 

 but afterwards fuses to form a single wide lateral main trunk. 



Host : Apodemus sylvaticus. 



Demodex nanus, sp. n. 



$ . A minute species very like that present in Sorex vul- 

 garis casteaneus. Length varying from less than 3 up to 

 slightly more than 3| times the width of the cephalothorax. 

 Abdomen considerably shorter than combined length of 

 cephalothorax-and capitulum. Capitulum usually much wider 

 than long ; the spines on its surface apparently obsolete or 

 absent. 



Total length 87-102 ft. 



Host : the black rat (Rattus rattus), a number of specimens 

 colled ed by the author from a freshly killed rat. 



Note. — Halm has already described a species of Demodex 

 (D. ratti) from a house-rat said to be Mus rattus. I have 

 not been able to consult his original description, which is 

 referred to by Gmeiner. The latter says the species is like 

 that of the dog. From this one would infer that it was an 

 elongated form of comparatively considerable size, similar to 

 that found in Rattus norvegicus. 



It is probable, indeed, that the rat from which Halm's 

 specimens were taken was really Rattus norvegicus, the brown 

 or Norwegian rat (syn. Mus decumanus). It is, of course, 

 possible that two species occur in Rattus rattus, as is certainly 

 the case in Apodemus sylvaticus. 



XII. — New Species 0/ Gerbillus and Taterillus. 

 By Oldfield Thomas. 



(Published by permission of the Trustees of the British Museum.) 



Gerbillus allenbyi, sp. n. 



A small species, with short feet and tail,' probably allied to 

 G. agag. 



General colour much more mouse-grey than the usual tone 

 of gerbils, markedly greyer than G. gerbillus • head, shoulders, 

 and most of the upper surface near " cinnamon-buff," but 



