200 REPORT—1846. 
op. cit. t. ii. (1837) p. 377. This tubercle is the rudiment of the mastoid 
process, which is so largely developed in birds, and which, in the echidna, 
overhangs the tympanic cavity. There is no glenoid articular surface upon 
the bone s and 16. We find, on the other hand, the squamosal under its proper 
mammalian form and connections, with a long and slender zygomatic process, 
and performing the function, peculiar to the class Mammalia, of supporting 
the mandible by the true glenoid articular surface in the echidna (fig. 12, 27). 
Dr. Kostlin, whose painstaking and minutely accurate description of the 
osteology of the vertebrate skull renders his conclusions as to their homo- 
logies worthy of respectful consideration, concurs with me in regard to the 
squamosal (27) of the monotremes, but regards the bone s-16 in the 
echidna as a dismemberment of the alisphenoid. In no mammal, however, 
do we find the alisphenoid concerned in immediately protecting the semicir- 
cular canals—this is the function of the petrosal: in neither mammal nor 
bird does the alisphenoid extend its connections so far back as to the basi- 
ex- and supra-occipitals. Inthe echidna, as in every other mammal and bird, - 
the alisphenoid (6) exists, exclusively exercising its essential function of trans- 
mitting the third division of the fifth pair by the large vacuity (¢) and with 
its normal connections modified only, as in the sheep and some other inferior 
mammalia, through the recession of the squamosal, by joining the mastoid, 
in addition to those which it unites with in man. I confess that I can perceive 
no other gain to anatomy by Dr. Kostlin’s new determination of s and 16 in 
the echidna as ‘hintere Abtheilung des Schlafenfligels’ or ‘hintern Schla- 
fenfliigel*’ (posterior alisphenoid), than an additional phrase to the synonyms 
of the mastoid. 
The discussion of the homologies of this bone under its modifications in 
the mammalia, and especially in the monotremata, will not be deemed super- 
fluous or too detailed, when it is remembered how valuable a key the cranial 
organization of the implacental monotremes with their bird-like heads becomes 
to the comprehension of the modifications of the cranial structure in birds 
themselves. If we pass from the comparison of the echidna’s skull, as re- 
presented in fig. 12, to that of the ostrich (fig. 8), we shall find there a bone 
(s) articulated in front to the alisphenoid (6), behind to the exoccipital (2), 
below to the basi-occipital and basi-sphenoid, above to the parietal 7, and 
coalescing by its inner surface with the petrosal. The sole modification of 
note in regard to connective characters, as compared with the mammalian 
petromastoid, is the loss of the connection with the squamosal, for which we 
have been progressively prepared by the conditions of that bone in rodents, ru- 
minants and monotremes. [n the bird this least constant element of the cranial 
walls (fig. 21, 27) has undergone a further degradation, is now dismissed en- 
tirely from any share in the formation of even the outer surface of the cranial 
parietes, and is reduced to its mere zygomatic form and function, serving 
exclusively to connect the jugal (fig. 21, 26) with the tympanic (2s); which 
function it performs in the echidna and in man, besides other superadded 
offices arising out of its peculiarly mammalian expansion into a scale-like 
lamina, or as compensatory of the reduction of the tympanic bone. Dr. 
Hallmann, however, in his elaborate monograph on the temporal bone, con- 
siders the bone s (fig. 8) to be the squamous or zygomatic element, and cites 
the following characters of the bone, in the young cassowary {, as establishing 
its homology with the squamosal :—“ its junction above with the parietal, in 
front with the alisphenoid and post-frontal and behind with the occipital: also 
its formation of the upper border of the meatus auditorius externus, and its 
* Op. cit. pp. 29, 126. 
+ Die vergleichende Osteologie des Schlafensbeins, p. 8. pl. 1. fig. 5. 
ow. 
