CLASSIFICATION OF THE SCIURID^. 233 



in Geosciurus and no longer than in FJuxeras ; but the upper sur- 

 face of the blade is very wide, much wider than the " handle " and 

 shaped like a spearhead, with convexly rounded sides and a 

 sliarp point, the width being rather more than half the length; 

 its dorsal median crest is much less pronounced than in Geosciurus 

 and its apex is not spatulate and hollowed. The inferior crest is 

 similar to that of Geosciurus. The bone itself measures 6 mm. 

 (Text-fig. 24, K-M.) 



Of the above-described bacula that of Atlantoxerus comes 

 nearest to the baculum of typical Sciurida? in the length of the 

 proximal cylindrical portion or handle ; Geosciurus comes next, 

 and Xerus and Euxerus last with the proximal portion quite 

 short. In the gradual differentiation or specialisation of the 

 blade the series runs Euxerus, Atlantoxerus, Geosciurus, Xerus ; 

 but which of the four is the most primitive type I am quite 

 unable to say. It is interesting, however, to note that the most 

 widely separated types are found in Xerus and Euxerus, which 

 until recently were referred to a single genus, although Geosciurus 

 and Atlantoxerus had been severed from Xerus. 



a. Distal portion of baculum a compressed blade uot expanded 



above, its upper edge narrow ISuxerus. 



a', lilade of distal portion of baculum lateral!}' expanded more or 

 less above, its upper surface spatulate or subspatulate with a 

 longer or shorter median crest behind. 

 b. The upper surface forming an elongated oval plate with uni- 

 formly convex sides, pointed at the apex and vrith a short 

 crest behind ; proximal portion of baculum only about half 



the length of the distal JTerws. 



b'. The upper surface much narrower, with sinuous sides and a 

 longer crest ; proximal portion almost as long as the distal 

 or longer. 

 e. Sides of the upper surface strongly constricted, the ter- 

 minal area apicall^^ rounded and hollowed Geosciurus. 



c' . Sides of the upper surface weakly constricted, the terminal 



area apically narrow and flat Atlantoxerus, 



A point of systematic interest connected with the glans penis 

 and baculum of the Xerinse is the complete absence of resem- 

 blance between them and the corresponding organs in the African 

 Squirrels, Protoxei'us, Eimisciurus. and Paraxerus, which were 

 associated with Xerus by Forsyth Major on the evidence derived 

 from skulls and teeth. 



5. The Sousliks and Marmots. 



The glans penis of Gitellus I have had no opportunity of 

 examining, but the bacula of several species are preserved in the 

 collection of the British Museum, In all cases the bone consists 

 of an elongated shaft, broad at its proximal end and narrowing 

 distally to the apex, which is upturned and expanded into a wide, 

 sometimes double disc with denticulated margin. 



The simplest type is the baculum of a specimen of G. mongo- 

 licus from Chifu, The shaft is abruptly expanded at the base, 



