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



It is evident that Geomys is more nearly allied to the families already described than 

 to the Cavies, and is not in any way so related to the latter, in the characters of its 

 otosteals, as is Bathyergus. 



In the Jerboas, Dipodidje, very interesting transitional characters may be observed. 



In one adult skull of Dipus cegyptius I have found the malleus and incus unanky- 

 losed (see PL LX. fig. 23) on both sides ; in other crania of this Jerboa these ossicles were 

 firmly united. The head of the malleus is extremely small, and deeply excavated by its 

 articular surface. The neck is slender and constricted ; there is a broad and distinct 

 lamina, along the front of which a portion of the head is prolonged as in Fiber (p h m). 

 The manubrium is very thin and fragile ; it is compressed laterally and broad at the base, 

 which has a curved upper border, ending externally in a true processus brevis (p h). 

 The outer aspect of the handle dilates into a very broad, spatulate, slightly recurved 

 extremity ; its inner border is stout, and has a sharp angle not far from the tip ; but the 

 tensor tympani muscle is not inserted into that projection, being attached to a little 

 tubercular processus muscularis where that border joins the neck (pm). The incus 

 of Dipus has a moderately developed body, a stout posterior crus, and a slender very 

 divergent processus longus. The stapes has long, exceedingly slender crura, not widely 

 divergent ; the base is narrow horizontally. I have found a canal between the crura in 

 some skulls of Dipus, but not constantly. 



Altogether the malleus of Dipus approaches to that of the Muridse, having a small 

 head and wide lamina, as in Mus, and a very broad manubrium, as in Fiber. In its 

 frequent fusion with the incus it allies itself to the Porcupines and Cavies, where, how- 

 ever, the head is always well-developed at the expense of the lamina. 



The malleus of Fedetes caffer (syn. FLelamys capensis) is more Hystricine, since the 

 head is much developed at the expense of the lamina, and the bone is firmly fused to the 

 incus. The stapes, too, has well-divergent crura. 



Among the Porcupines, Hystricid^i, the fusion of the two outer ossicles is extreme, 

 and the line of separation is barely perceptible externally, in Hystrix ; but in none of 

 the rodents to be described does this bony union extend beyond the normal articular 

 area as in Bathyergus. 



The malleus of BZystrix cristata (PL LX. fig. 24) has a very well-developed head, 

 which projects very prominently forwards, taking the shape of a Phrygian cap, the 

 convex border being above, the concave below ; the blunt apex points forwards and 

 downwards. In a perfect specimen the processus gracilis (p g) may be seen running close 

 to the lower border of the head, a very limited area of lamellar bone (I) connecting 

 them. The manubrium, much flattened laterally and very fragile, is trapezoidal rather 

 than triangular, on account of the sharp angle on its inner border, which is here a true 

 processus muscularis, the tendon of the tensor tympani being actually inserted into it. 

 The broad upper border of the base, well-developed here, as in Fiber or Dipus, forms 

 a very obtuse angle with the outer border, so that the site of the processus brevis 

 (pb) is not so prominent as the angular processus muscularis which is readily 

 mistaken for it in a detached bone. The outer border dilates very gradually into a 

 narrow, spatulate, slightly recurved extremity. The incus, which is firmly ankylosed 



