OF THE MAMMALIAN OSSICULA AUDITUS. 453 



The incus of B. mysticetus (PL LXII. fig. 29) and of B. australis, which exactly 

 resembles it, is more generalized than in any other cetacean (excepting, perhaps, Mega- 

 ptera and Neobalcena). Prom the position of the articular surface of the malleus, the 

 incus lies above and internal to it, with the corresponding surface downwards. The body 

 is very solid and square. The posterior cms is short; stout, and recurved at its extremity. 

 The processus longus, not so over-developed as in other Cetacea, is still very stout and 

 broad to its extremity; it forms a bold sigmoid projection. A faint groove cuts off the 

 Sylvian apophysis from the rest of the processus longus ; it does not expand or project in 

 any way from the crus itself. The articular surface on the body is very prominent ; its 

 upper facet is almost plane and larger than the lower, as in the malleus ; a sharp ridge, 

 which separates them, fits into the groove between the facets of the malleus. 



The stapes (PL LXII. fig. 29) is nearly half an inch long and very narrow. The head 

 is well developed, and bears a rough surface posteriorly for the insertion of the stapedius 

 tendon. The crura are long, straight, and slender ; they diverge but little, and are con- 

 nected by a lamella, which has only a very small aperture in it ; this lamella is evidently 

 an ossification of the thin membrane thrown across the crura in the recent stapes of a 

 typical mammal. The base is generally immovably soldered to the fenestra ovalis *. 

 The stapes of Balcena bears a certain resemblance to that of Stenorhynchus, and differs 

 from all other Cetacea in the length and slenderness of its crura. 



In describing the process which unites the malleus to the tympanic bone, the anterior 

 column or border of that process (PL LXII. fig. 28, PL LXIII. fig. 13) was assumed to 

 represent the true processus gracilis, and the posterior (phm) as homologous to the 

 prolongation from the head of the malleus, which in other mammals generally joins it. 

 This posterior column runs as an elevated ridge two or more inches over the outer 

 surface of the scroll-like tympanic bone, ending in a broad lamina, which is closely 

 applied to the upper part of the deep oblique groove which separates the anterior from 

 the posterior portion of that bone, which would seem to indicate that the posterior 

 column was the processus gracilis. But the anterior, which seems, from its position 

 between the head and manubrium, to be the true processus gracilis, is soon lost on the 

 inner side of the tympanic bone, where it makes a sharp ridge in some whales (Phy- 

 seter &c), as if the root of Meckel's cartilage had become embedded in that bone during 

 ossification. Altogether, then, arguments are in favour of the anterior column being the 

 homologue of the processus gracilis. This can alone be verified by the examination of 

 very small foetal Cetacea. 



In the Pike Whale (Balcenoptera rostrata) the auditory ossicles assume much more 

 thoroughly the type most frequent in the Cetacea, excepting Balcena. 



Indeed the malleus (PL LXII. fig. 30) is as extreme in its modifications as that of 

 Globiocephalus, Phoccena, and Belphinus, being only exceeded in specialization by the 

 Ziphioids. It is less like the malleus of Balcena than is the same ossicle in Physeler or 

 even Orca. 



* In Balcena mysticetus ; but I have since found this not to be invariable, and in an adult petrous bone of 

 B. australis in the College Museum the stapes has fallen out. 



SECOND SERIES. — ZOOLOGY, VOL. I. 3 P 



