OF THE MAMMALIAN OSSICULA AUDITUS. 409 



Squirrels, and ends in a very narrow, spatulate, and not discoidal termination. There is 

 no trace of a processus brevis. The body of the incus is well developed, the anterior 

 part as much as the posterior. The stapes differs from that of the true Squirrels ; its 

 crura are short, stout, and almost straight, not very divergent. There is no intercrural 

 canal in the recent skull. The base is rather thick, quite flat towards the vestibule, and 

 projects more than a millimetre beyond the insertion of both crura. In the general 

 characters of its ossicula, Anomalurus is as nearly a true Squirrel as it has recently been 

 shown to be, in its general anatomy, by Mr. E. R. Alston *. 



In the Plying Squirrels (Pteromys, PI. LX. fig. 7) the malleus, in general resembling 

 the British Squirrel's, has a more distinct trace of the processus brevis on the manubrium 

 even than Spermophilus. The termination of the handle is spatulate. The body of the 

 incus is as shallow as in Tamias, Spermophilus, and many of the smaller species of Sciurus. 

 The stapes is of the same form as in the above-named genera, and unlike that of Anoma- 

 lurus, having slender, widely divergent crura, occupied in lifetime by a bony canal ; the 

 anterior crus is inserted into the base some distance from its extremity, the posterior 

 joining the base almost precisely at the opposite end. 



The above descriptions show that Sciurus is the most specialized of the genera already 

 examined, particularly in the absence of any processus brevis to the malleus, which is 

 not as broad as in most Rodents. The distinctions from Sciurus in the other genera 

 already discussed will be seen to be truly transitional when allied families are 

 considered. 



In the Marmots (Arctomys, PI. LX. fig. 11) the sides of the malleus are flatter than in 

 Sciurus, and the head rises into a conspicuous blunt point high above the articular surface, 

 which is extremely deep and narrow. All this gives a peculiar aspect to the ossicle. 

 The manubrium is much broader at the base than in any true Squirrel, though other- 

 wise of the same type ; the extremity is very faintly spatulate, the inner border very 

 angular. The neck is quite wanting; and there is hardly any trace of a processus 

 brevis. The incus has a very wide shallow body ; the crura are very similar to those 

 in Sciurus. The stapes is a proportionally very large bone, with slender, bowed, and 

 widely divergent crura. There is a dense bony canal between them during lifetime ; and 

 they are inserted almost exactly into the corresponding extremities of the base, which is 

 very narrow vertically, and slightly convex towards the vestibule. The characters of the 

 malleus and stapes separate Arctomys from the Squirrels : in the former bone this genus 

 approaches the Beaver, where, however, there is a neck and a very prominent processus 

 brevis. The Marmot's malleus, with its broad, flattened manubrium, is of the type most 

 prevalent in this order. 



In the Dormice, Myoxume (PI. LX. fig. 13, Myoxus glis), the head of the malleus is as 

 compressed laterally and as flattened at the top, above the articular surface, as in Sciurus, 

 and the articular surface is as deep and narrow vertically. So far it differs from 

 Arctomys, where the head rises high and the articular surface is wide and shallow. How- 

 ever, the malleus of Myoxus resembles families yet to be described in having a faint 



11 On Anomalurus, its Structure and Position," Proc. Zool. Soc. 1875. 



