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 to a 

 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, s) presents a most interesting and instructive combination of both the 

 modification of expansion and of that of direct union with the alisphenoid (e), 

 which is here effected by the mastoid plate independently of the petrosal (ie). 

 In fig. 12 these characters are well exposed by the removal of the squamosal 

 27, and tympanic 2s, which retain their primitive independence throughout 

 life in the echidna. If now we compare the bone s and 16 with the carti- 

 laginous and osseous mass 8 and ie 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 i6 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 bone 8 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 ' Lecons d' Anatomie Comparee,' 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 veritable temporal, qui n'aurait pour toute 

 apophyse zygomatique qu'un tres petit tubercule pres de la facette glenoide," 



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 

 trouve quelque rapport entre l'apophyse masto'ide qui, dans la plupart des animaux, appar- 

 tient a l'occipital, et l'apophyse transverse de 1'atlas et des autres vertebres ; sur quoi il faut 

 remarquer que ces rapports sont moindres dans l'homme a certains egards que dans les qua- 

 drupedes, puisque 1'atlas n'y a ordinairement qu'une echancrure pour le passage de l'artere, 

 et que l'apophyse masto'ide y appartient entiere au rocher." — Resume sur le question — ' Le 

 crane est-il une vertebre ou un compose de trois ou quatre vertebres ?' Lecons d' Anatomie 

 Comparee, t. ii. (1837) p. 711. 



* In the article ' Monotremata,' Cyclopaedia 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. 



