MORPHOLOGY OF THE VERTEBRATE SKULL 30 
tohyal’ of the crocodile. But, if this homology be granted, the 
very superficial resemblance to the malleo-meckelian connection 
in mammals is purely accidental and of no homological signifi- 
cance. Versluys indeed finds (’03, p. 177) that this secondary 
connection by means of the hyoid, is the only way in which the 
Sauropsid extracolumella is ever connected with the Meckelian 
cartilage. In brief there can now be little doubt that the malleo- 
meckelian rod of mammals represents solely the first or mandib- 
ular visceral arch and has nothing to do with the second arch 
which is the source of the extracolumella and hyoid cornu (cf. 
Howes and Swinnerton, p. 49). Versluys (’03, p. 177) concluded, 
in opposition to Peters and others, that the extracolumella and 
suprastapedial, instead of giving rise to the malleus and incus 
have practically disappeared in mammals and are only repre- 
’ sented by certain transitory embryonic vestiges connecting the 
stapes and hyoid (’03, Taf. 11, figs. 3, 4.) 
With regard to the origin of the mammalian tympanic mem- 
brane, it seems likely that at least some of the Cynodonts already 
approached mammalian conditions. In the remarkably mammal- 
like genus Sesamodon of Broom (fig. 25) the auditory groove 
(doubtless homologous with that of mammals) indicates essen- 
tially mammalian conditions for the tympanic cavity and mem- 
brane. On the other hand, in the far more primitive Cynodont 
Bauria there is little hint of mammalian structures and the tym- 
panic membrane, if differentiated, was probably stretched as in 
reptiles behind the squamosal and articular. 
The stapes of Bauria is supposed to have touched the quad- 
rate; but conceivably it may also have been connected with an 
extracolumella; just as in embryo lizards the stapes-extracolu- 
mella chain touches the quadrate (Versluys ’03, Taf. 8, fig. 8); 
the hyoid was perhaps still connected with the extracolumella. 
The essential feature of a primitive auditory chain is a jointed 
system of rods, subjected to pressure at opposite ends but kept 
tense by muscular pull and by direct fastening to adjoining bones. 
In both mammals and reptiles the outer end of the chain is con- 
nected with the tympanic membrane. But is the tympanic 
membrane homologous in the two classes? 
JOURNAL OF MORPHOLOGY, VOL. 24, NO. 1 
