OF THE MAMMALIAN OSSICULA AUDITUS. 447 



At first sight the anatomist may be inclined to consider that pm (fig. 20) is accessory, 

 and acpm the homologue of the processus muscularis; for acpm lies in the usual 

 position occupied by that process in laminated mallei ; and though the muscular process 

 is often found lower down, as in the Cavies, where it is seated on the manubrium, it is, 

 on the other hand, rarely seen higher up than acpm. But whilst I have never 

 found both processes coexisting in the malleus of any other Bat *, the upper of the two 

 observed in Asellia {pm, fig. 20) is constantly observed in the remaining insectivorous 

 Chiroptera (PL LXII. figs. 22, 24, 25, pm). As, then, the lower process, acpm, in 

 Asellia, is the peculiarity of this (and probably of a few allied Bats), and the upper is the 

 most frequent in others, it is reasonable to consider the latter, pm, as the homologue of 

 the processus muscularis, so common in other orders, the lower acpm being an acces- 

 sory process. From the base of acpm a sort of splint or buttress runs for a short 

 distance across the lamina ; it must not be mistaken for the thin processus gracilis below 

 it, though it is most probably ossified as a part of that element. 



Besides the structures just described, a large orbicular apophysis exists, identical in 

 position to the same in Sorex and Mus, and as large in proportion to the entire bone as 

 in those animals, but less conspicuous, because it is sessile, not pedunculated. It is far 

 better developed than in Pteropus. 



The processus gracilis is slender, and appears as the thickened free border of the 

 lamina; it is joined by a process from the head, the homologue of what has been 

 already very frequently described in this memoir ; the two form a rather broad curved 

 process, which fits into a cleft in the ring-like tympanic bone. The lamina is very 

 broad, and formed entirely of semitransparent bone ; a buttress from the lower processus 

 muscularis runs across it, but is lost before the processus gracilis joins the process from 

 the head. 



The manubrium is slender and long, well compressed laterally, and narrow, in that 

 sense, even at the root. It is not triangular, as is the case in most laminated mallei, but 

 truly quadrilateral ; for the inner aspect as well as the outer forms a narrow surface 

 sharply bordered from the sides almost to the point. A tendency to this condition may 

 be observed in the malleus of the Shrew and in some Carnivora, but never to so complete 

 an extent ; for in Asellia this inner surface is as broad as the outer. In the great majority 

 of animals the inner aspect of the manubrium is a simple border, sharp or blunt as the 

 case may be. The outer border or, rather, surface is very narrow, but ends below in a 

 distinct spatulate dilatation f. Above it forms a rather sharp angle, slightly incurved 

 towards the neck, in fact a rudimentary processus brevis. 



In the well-formed lamina and orbicular and muscular process the malleus of Asellia 

 tridens resembles that of certain Insectivora and Rodents, and differs from the less 

 specialized type observed in Pteropus. It must be borne in mind that the characters of 

 the malleus in Asellia and other insectivorous Bats almost coincide with the distinguish- 

 ing features of the same ossicle in two tribes of mammals belonging to different Orders, but 



* Excepting Phyllorhina fulva and P. speoris, where the malleus in all other points precisely resembles that of 

 Asellia tridens. f This dilatation is shown in fig. 21, the malleus of Plecotus. 



3o 2 



