200 report — 1846. 



op. ciL 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 ic 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-ig 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. In the 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 (tr) 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 8 and 16 in 

 the echidna as 'hintere Abtheilung des Schlafenfltigels' or 'hintern Schla- 

 fenflugel*' (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 (r>), 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. In 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 f, 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. 



t Die vergleichende Osteologie des Schlafensbeins, p. 8. pi. 1. fig. 5. 



