1898.] OF THE EAMILT AKOMALUBID^. 453 



The molars in the present specimen are much worn, but there 

 is no doubt they are of a very simple form, having a single enamel 

 fold on the outer side only, dividing the tooth into two shallow 

 oval cups, and thus would not differ greatly in pattern from the 

 teeth of Pedetes except in the fact of their being brachydont 

 instead of hypsodont. 



The incisors are A^ery large, being little inferior in antero-posterior 

 depth to tho«e of the large squirrels of the Stangeri group. The 

 molar series are in parallel rows, the teeth very small and simple, 

 as already stated ; the first and last teeth of the series are abont 

 equal in size and little more than half the size of the two middle 

 teeth, which are also about equal one to another in size. The 

 teeth in the lower jaw, both the incisors and molars, bear the same 

 relative proportions one to another. 



The formation of the mandible (Plate XXXV. fig. 5) resembles 

 that of Idiurus, as described by Mr. Miller, in the formation of 

 a thickened bridge between the coronoid and condylar processes, 

 with a thin, oval, almost transparent plate of bone beneath it. 

 From the figure given of the mandible of Idiurus macrotis it is 

 impossible to follow the form of the incisors, but in our new genus 

 these teeth originate immediately beneath, or in the base of, the 

 coronoid process, being therefore widely different from Anoma- 

 lurus, in which genus these teeth germinate externally on a level 

 with the last molar. 



Type in British Museum. No. 98.5.4.6. 



S . Benito Eiver (15 miles from mouth), 22nd Feb., 1898. 



Measurements taken in the flesh : — Head and body 203 millim.; 

 tail 167 ; hind foot 40 ; ear 22. 



Fang name, osin. "Caught in the hands, in a hollow tree ' 

 ((?. L. Bates, collector). 



Measurements of Skull : — Greatest length 46 milhm.; basal length 

 39 ; zygomatic breadth 25-5 ; length of frontals 17 ; intertemporal 

 constriction 7"5 ; length of nasals 13 ; greatest breadth of nasals 5 ; 

 tip of nasals to gnathion 13*5; height of infraorbital foramen 10-5, 

 breadth 5*7; diastema 11*5; antero-posterior depth of incisors 4 ; 

 length of upper tooth -row 6 ; breadth between msl 2 ; breadth of 

 palate in front of molar series 3 ; length of auditory bulla 7'2 ; 

 mandible, greatest length (bone only) 29, greatest depth 18 ; tips of 

 incisors to condyle 34*5 ; back of incisors to coronoid 22, to condyle 

 29, to angle 22*3 ; length of lower tooth-row 6. 



The great power and depth of the facial portion of the skull, the 

 relative size of the teeth and form of the zygomatic processes of 

 tlie maxillae, the shape of the infraorbital foramina, the narrowness 

 of the palate, and strength of the lower jaw are characters in which 

 Aethurus resembles Idiurus ; and the peculiar and highly special- 

 ized form of the tail, in which it resembles Anomalurus, places its 

 affinity with that genus beyond doubt. On the one hand, therefore, 

 we have cranial, on the other external characters of resemblance. 



Unlike either of these genera, Aethurus possesses no flying- 

 membranes, and the skull differs markedly in the frontal region. 



