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



tremely small ; and hence the greater narrowness of the malleo-incudal articulation, as 

 seen from within, compared with Echidna. But it sends up from without a scale-like 

 process (shm), as in that animal, which conceals the body of the incus and the root of 

 its long process ; so that on viewing the bones from within, tbe same appearance of a 

 lamina of bone, reflected from that process to the malleus, may be noted. When the 

 incus can be detached, a smooth slightly concave space on the process from the head 

 comes into view. This has been already figured in Peters's memoir already quoted. 



From the head a very stout and straight process is sent forwards, blending with the 

 very thin papery processus gracilis. The neck is longer, and much better defined than 

 in Echidna, and forms a very wide curve. The true processus folianus is merely the 

 slightly thickened free border of the narrow but distinct lamina; it joins the well-deve- 

 loped process from the head. It is only by a careful comparison of these elements in 

 Ornithorhynchm with the same in laminated mallei of placental mammals that the error 

 can be avoided of taking the process from the head for the processus gracilis *. 



The processus muscularis is of a unique form. It is a wide angular projection from 

 the inner side of the neck above the manubrium. The processus gracilis (and its adjunct 

 just referred to) is united with the tympanic bone in the same manner as in Echidna. 

 This bone is here less developed aud more sharply curved than in that animal. Hence, 

 united with the malleus, it bears a very characteristic distinctive appearance. 



The manubrium is short, ill developed, and so imperfectly ossified, as to shrink when the 

 malleus is dried. Its outer angle is blunt, very broad, and slightly concave, somewhat 

 as in Phascolarctos. Its shank is very slender, and slightly dilated at the extremity; 

 the outer aspect is excessively narrow, but distinctly bordered from the sides. 



The incus is small and flattened, as in Echidna. Its inner surface is more concave, and 

 narrower where it joins the malleus. The crura are short, stout, and straight, and almost 

 of equal length ; the stapedial cms bears a very faint indication of a Sylvian apophysis. 



The stapes is absolutely columelliform, and not so firmly osseous as in Echidna ; the 

 column is rather longer, the head broader, and the almost circular footplate is not 

 quite as large. As a processus longus of the incus exists, the column must be the 

 exact homologue of the same in Echidna, and of the two crura of the stapes of the 

 higher mammals, and, for the same reason, represents only part of the column of the 

 Sauropsidan columella. In fact the presence of the processus longus of the incus throws 

 the stapes of the Monotremata both towards other mammals, and away from the birds. 



The result of the above considerations shows : — 



1. That, in their ossicula, the Monotremata are as low as in the rest of their skeleton, 

 yet, as might be expected, thoroughly mammalian. 



2. That their distinguishing features are a peculiar form of articulation between the 

 malleus and incus by means of a scale-like development from the head of the former, 

 and the presence of an absolutely columelliform or unicrurate stapes. Otherwise the 

 three ossicula are not much modified from their representatives in the lower Marsupials. 



* Compare phm unipg in PI. LXIV. figs. 36 & 3S, with the same in PI. LXIII. fig. 14. 



