s 
ON THE VERTEBRATE SKELETON. 199 
connections of the mastoid, which results from the gradual withdrawal, in the 
mammalian class, of the squamosal from the proper cranial walls. With much 
inconstancy of relative size in the mastoid, of which the dugong and the walrus 
offer two extremes, we discern upon the whole a progressive increase in de- 
scending through the mammalian class: in the walrus, for example, the mastoid, 
or petromastoid, forms as large a proportion of the outer lateral walls of the 
cranium as does the squamosal; and, in the sheep, the removal of the squamosal 
exposes the connection of the petromastoid with the alisphenoid,—a return toa 
relation common in the oviparous vertebrata: it is shown from the inner side 
of the cranium in the sheep, in fig. 7, 16 and 6. The mastoid of the echidna 
(fig. 12,8) presents a most interesting and instructive combination of both the 
modification of expansion and of that of direct union with the alisphenoid (6), 
which is here effected by the mastoid plate independently of the petrosal (16). 
In fig. 12 these characters are well exposed by the removal of the squamosal 
27, and tympanic 23, which retain their primitive independence throughout 
life in the echidna. If now we compare the bone s and i6 with the carti- 
Jaginous and osseous mass s and 16 in the skull of the human embryo (fig. 11), 
and allow for the change produced in the position of the alisphenoid (6) by 
the gradual withdrawal of the squamosal (27), traceable in the intervening 
forms of mammalia, the special homology of the petromastoids at the two ex- 
tremes of the mammalian class will be obvious and unmistakeable. The bone 
s and 16 in the echidna, fig. 12, is connected below and behind with the basi- 
occipital and exoccipital (2), behind and above with the supraoccipital (3) and 
parietal (7), in front with the tympanic, the squamosal, and also, as a conse- 
quence of the modified position of the latter and of its own increased deve- 
lopment, with the alisphenoid (6). - All the connections, save that with the 
alisphenoid, are identical with those of 8 and 16 in the human embryo; and 
the supervening alisphenoidal connection in the echidna affords an additional 
light to the determination of the bone in the lower vertebrata, since it is a 
.consequence‘of the progressive advance to a lower (oviparous) type, in the 
descent through the mammalian scale. In regard to the essential functions 
of the petromastoid, we find the petrosal portion inclosing the membranous 
labyrinth, and the mastoidal portion giving exit to the blood from the great 
lateral venous sinus and supporting the tympanic*. It will be unnecessary 
to dwell further on the broad and obvious characters by which the homology 
of the bones and 16 in the echidna is established with the equally independent 
petromastoid in the sheep and walrus, and with the petromastoid portion of 
the human ‘temporal bone.’ 
_ The continuators of the ‘ Legons d’ Anatomie Comparée,’ influenced by the 
large proportional size of the petromastoid in the echidna and the share 
which it consequently takes in the formation of the cranial parietes, supposed 
it to be the squamosal:—‘“le véritable temporal, qui n’aurait pour. toute 
apophyse zygomatique qu'un trés petit tubercule prés de la facette glénoide,” 
higher law of general homology may be learnt from the application by Cuvier of his idea of 
the mammalian mastoid to the refutation of the vertebral theory of the skull. ‘On a aussi 
trouvé quelque rapport entre l’apophyse mastoide qui, dans la plupart des animaux, appar- 
tient a l’occipital, et ’apophyse transverse de l’atlas et des autres vertébres; sur quoi il faut 
remarquer que ces rapports sont moindres dans l’homme a certains égards que dans les qua- 
drupédes, puisque l’atlas n’y a ordinairement qu’une echancrure pour le passage de |’artére, 
et que l’apophyse mastoide y appartient enticre au rocher.”—Resumé sur le question— Le 
erane est-il une vertébre ou un composé de trois ou quatre vertébres?’ Lecons d’Anatomie 
Comparée, t. ii. (1837) p. 711. 
* Tn the article ‘ Monotremata,’ Cyclopedia of Anatomy and Physiology, 1841; influenced, 
then, by the absence of the external character of the process, I described the petromastoid as 
the petrous bone. 
Pa 
