304 . PEOF. W. K. PAEKEE ON THE STEUCTUEE AND 



I need not show the reader how all these modifications tend towards, or illustrate, 

 what we are familiar with in the auditory apparatus of Man, and the Mammalia, 

 generally. But there are some things worth especial notice. 



In the Insectivora (e.g. Mole and Hedgehog) the basisphenoid gives off wings to 

 enlarge the tympanic cavities ; in the Marsupials the alisphenoids do the same ; whilst 

 in others (notably many of the Carnivora) a distinct, hollow lunule of cartilage appears 

 on the outside of the parachordals : it is the innermost segment of the external ear, 

 and helps the more superficial " os tympanicum" to form the drum-cavity'. Pneuma- 

 ticity of the hind skull is very variable in Mammals ; it is very much developed in 

 Petaurus sciureus. In some Mammals large air-cavities are developed, retrally, in the 

 base of the skull, from the hind part of their extensive nasal labyrinth ; these burrow 

 the skull-base in the same way as the Eustachian tubes do in the Crocodile. 



In the Mammals the first and second visceral arches are formed after the hyostyUo 

 pattern of the Skate, but are much specialized. Both the malleus and the incus lie 

 under the tegmen tympani. The part answering to the cerato-branchial (Meckel's 

 cartilage) is still large in Manis, at birth, and in that stage it rapidly re-enlarges from 

 the sharp end of the " processus gracilis," and lies inside the hind part of the dentary. 

 The stapes is not a periotic element ; the incus is either a second " pharyngo-hyal " 

 added to the stapes, or the quadrate in an arrested state; the "epihyal" end of the 

 main bar is ossified as the " tympano-hyal" (Flower); the rest of that bar is the 

 " ceratohyal," which finishes below as a "hypohyal" to articulate with the common 

 " basihyo-branchial." The little "interhyal" (= infrastapedial of the Sauropsida) is 

 detached and carried away by the perfectly specialized stajjedius muscle, to whose 

 tendon it becomes attached, and thus gets to be united t'j, and often ankylosed with, 

 the neck of the stapes ^. 



I put these things down as they appear to me from my present standpoint ; anyhow 

 the study of the development of the Crocodile's skull is very profitable, and from it -I 

 hope to borrow much light in a renewed investigation of the Mammalian skull in its 

 various ordinal modifications. If the Crocodile does not lie directly below the Mammals, 

 it nevertheless shows us how the Mammals may have arisen from some generalized 

 oviparous form. 



' There is a division between the two parts of the " cavum tympani " in the Carnivora : the bony and 

 cartilaginouB anmdi are not distinct in most Mammals, but the bone-cells soon transform the cartilaginous 

 lunule into a bony ring; outside this, the inner part of the concha is more or less segmented. 



' That little segment, with the stapes itself, would appear to be all that the Mammal shows of the stapedial 

 chain of the oviparous types ; Professor Huxley's terms for the parts of that chain (namely, stajjedial, mediu- 

 stcijiedial, &c.) are of permanent value, being morphologically accurate. 



