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



bones, lie must, for the same reason, place the Porcupines and Cavies nearly as low in 

 the scale, and decidedly in an inferior position to the Marsupials ; and as for the stapes, it 

 is of a type precisely imitated in the placental Manis. 



It remains for me to describe the ear-bones of this Order as I have detailed the same 

 in higher milk-secreting vertebrata, investigating carefully their exact homologues in 

 the former. 



In Echidna hystrix the two outer ossicula are proportionally much larger, better 

 developed, and more firmly osseous than in Omithorhynchus. It is on that account 

 advisable to commence with the description of the ear-bones of the so-called Porcupine 

 Ant-eater. 



Let the united malleus and incus of an Echidna be removed from the skull. In so 

 doing observe that the processus brevis of the incus lies anterior as well as superior to 

 the stapedial eras in the natural condition, an exaggeration of what is seen in the 

 Sirenia, Marsupials, and other mammals. Even in Homo the short crus is almost as 

 superior as it is posterior to the rest of the incus. Observe also, if the skull be im- 

 perfectly macerated, how large the tensor tympani muscle is, as Professor Huxley * 

 has already remarked. Were it attached to the inner edge of the manubrium, as in 

 the Bodents, it would require still greater care and preparation, to remove the malleus 

 entire, than even is necessary in those animals. Now let the united malleus and incus 

 be examined from the inner aspect (PL LXIV. fig. 36), and the complete outline of the 

 incus may plainly be seen. The inner aspect of its body is free, square, and faintly 

 concave, whilst antero-inferiorly it is limited by a convex border fitting into a concavity 

 in the malleus. The whole, at this region, has precisely the same appearance as may be 

 seen in articulating the incus and malleus in a Marsupial, or in any mammal where the 

 latter ossicle has a small ill-developed head. 



But view the bones from without (fig. 35), and it will then be observed that the 

 malleus encroaches on the whole of the outer surface of the body of the incus, so that 

 only the processus brevis (pb) and the tip of the processus longus (pi) can be seen. As 

 it covers the greater part of the long process, on turning to the inner aspect of the 

 ossicles the singular appearance of a lamina of bone, reflected from the outer border of 

 the stapedial crus towards the manubrium, is at once accounted for, that lamina being 

 simply an integral part of the malleus appearing from behind at the point where, from 

 this aspect, the incus ceases to cover it. 



Hence the incus is articulated to the malleus by its whole inner surface, including the 

 greater part of its long crus, and also by its convex inferior border. It is firmly anky- 

 losed to the malleus ; and even in a young skull the ossicles cannot be separated unin- 

 jured without difficulty. 



Bearing in mind the above remarks, the description of the malleus by itself will now 

 be more comprehensible. The head is narrow and concave ; externally it is prolonged 

 upwards into a thin scale (PI. LXIV. fig. 35, shm), of which the wide convex free 

 border reaches to the base of the short crus, and conceals still more of the long process 



* Proc. Zool. Soc. 1869, " On the Representative of the Malleus and Incus of the Mammalia in the Sauropsida." 



