418 MR. A. H. G. DORAN ON THE MORPHOLOGY 



The stapes has perfectly straight and well-diverging crura ; the hase is broad, and 

 projects beyond their insertions. There is no bony intercrural canal in the recent skull. 



In the Eabbit (Lepus cuniculus, PI. LX. fig. 34) the ossicles are identical in form with 

 those of the Hare. In both these animals the processus muscularis may be situated 

 absolutely on the neck of the malleus, and not at all on the inner edge of the manu- 

 brium as is usually the case. 



The conclusions arrived at, after the study in detail of the ossicula of the Rodentia or 

 Glires, is as follows : — 



I. In the Sciuromorpha there is a tendency in the malleus to suppression of the neck 

 and processus brevis. This is complete in Sciurus and Anomalurus ; but the neck and 

 the process are gradually developed in Xerus, Tanrias, Spermophilus, Pteromys, Myoxus*, 

 and Castor, the process being perceptible as a slight projection in the first four genera, 

 whilst it is well developed and there is a distinct neck in the Dormouse and Beaver. 

 In Arctomys there is no neck, and the processus brevis is as ill developed as in the 

 Ground-squirrels and Pteromys ; the malleus is extremely flattened, and projects above 

 the articular surface. As in all other rodents where the incus is free f, it has a rather 

 square solid body, with the posterior crus but little divergent. The stapes is large, with 

 wide, slender, divergent crura (least so in Anomalurus) ; a bony canal runs between the 

 crura in lifetime. 



The great breadth of the manubrium, so distinctive of Rodents, is well marked in 

 Castor and ArCtomys, least in Scmrus. The processus muscularis is situated on its 

 inner border. 



II. In the Myomorpha the laminated malleus reappears in its best-developed form in 

 Mm, Sydromys, and allied species ; in some of these Muridse there is an orbicular 

 apophysis, so frequent in the Chiroptera and the Shrews. In the less-typical genera the 

 lamina is narrow, the manubrium extremely broad, with a prominent processus mus- 

 cularis on its inner border. The stapes has long slender crura, little curved or diver- 

 gent. A bony intercrural canal is rare. 



III. In the fossorial Rodents remarkable varieties are seen in all the ossicula, especially 

 in the stapes, which is bullate in Geomys and of unique type in Spalax. Mlobius, Spalax, 

 Rhizomys, and Geomys J show distinct affinities to the Murida? in the form of the 

 malleus, whilst in JBathyergus we find that the malleus and incus are fused even to a 

 greater extent than in the Hystricidse. 



IV. In the Hystricomorpha the malleus is almost invariably fused to the incus in the 

 adults, but often free in young subjects. In Bipus § the malleus is more like those of 

 some of the Muridse than is that ossicle in the rest of the group. The head of the 

 malleus is produced forwards moderately in Hystrix and its allies, also in Cavia, 



* Myoxus is rather Soiuromorpliic than Myomorphic in its ossicula. 

 t Excepting Sjxdax and its allies. 



J The absence of a neck to the malleus in Geomys allies it to the Squirrels. 



§ As the malleo-incudal ankylosis is frequent in Bipus, and probably constant in Pedetes, these genera are not 

 placed here among the Myomorpha. 



