82 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the stapes, which connects with an incus, and lastly comes the mal- 

 leus, which has two long processes, a manubrium which is inserted in 

 the tympanic membrane, and a processus anterior (Folian process) 

 which extends into the petrotympanic (Glaserian) fissiure of the 

 temporal bone. That these parts are not to be compared to the 

 columella of the sauropsida and anura is shown by the fact that they 

 invade the tympanic cavity from in front and that they are in front 

 of the chorda tympani nerve, the columella of the non-mammals 

 lying behind it. 



The homologies of these parts seem clear. Jn development the 

 malleus is the posterior end of Meckel's cartilage, being in the posi- 

 tion of the articulare of lower groups. It 

 articulates with the incus, which in turn 

 at first articulates with the wall of the 

 otic capsule, as well as with the stapes, 

 and thus corresponds to the quadrate. 

 The stapes is apparently the same through- 

 out the whole of the tetrapoda. It is to 

 be noted that many paleontologists deny 

 the homologies recognized here, think that 

 in the mammals the quadrate has been lost 

 in the glenoid fossa, and find the malleus 

 and incus in the columella. For this they 

 Fig. 8i.— Early chondro- ^ave no evidence except comparisons with 

 cramum of Ammocoete stage certain theriomorph reptiles. The litera- 



of Petromyzon, after Schneider. i_ • i • . 



h, hyoid; nc, notochord; oc, otic ture, which IS extensive, should be con- 

 tSecuL.^''' P^'^^^^'"'^^^' "■' suited for details. 



The Skull in the Diferent Classes 



CYCLOSTOMES have only the cartilage skull, and this can be homologized 

 only in part with that of other vertebrates; indeed the skulls of the two groups of 

 cyclostomes are not easily compared. The peculiarities are in part due to the 

 development of a suctorial mouth with its necessary framework. The chondro- 

 cranium of the Ammocoete stage of Petromyzon is readily understood. Para- 

 chordals, otic capsules and trabecule (fig. 8i) are normal, but a pair of ventral 

 horns (A) are problematical. Their position in front of and below the otic cap- 

 sule renders doubtful the interpretation of hyoid or quadrate sometimes given 

 them. 



The adult Petromyzon (fig. 82) has a typical brain trough, roofed by a 

 slender synotic tectum and fibrous tissue, and closed in front by the unpaired 

 nasal capsule, bound to the rest by fibrous tissue. The cranium is continued 



