TRANSACTIONS OF THE SECTIONS. 91 
taken for the thoracic or anterior extremity, and called by ichthyologists, judging 
from external character only, the pectoral fin, was really the coxal segment and 
leg, and not the humeral arch. The author presented a sketch copied from the Lec- 
_ tures on Comparative Anatomy by Prof. Owen, to show how the mistake had arisen. 
. The object of the diagram was to show that in the human foot, the analogy to the 
human arm may be traced by merely elongating the calcis and scaphoid bones, so 
as to represent the ulna and radius, while the astragalus may typify the very com- 
_ pressed humerus; in the fish the foot is turned with the sole or palmar aspect for- 
wards, consequently what is the internal malleolus in man and mammals becomes 
the external in the fish, and so very much developed, that it forms in the osseous fishes 
the larger part of the tibia, meeting almost with its fellow from the opposite arch 
_ under the mesobranchial or hyoid region, and called by Cuvier the scapulo-clavicu- 
laire, and by Owen the coracoid, having the fibula more internal and called epico- 
racoid by Owen. 
Should the Section agree to this view of the pectoral being the coxal instead of 
the scapular or respiratory extremity, they will perceive that it cannot be the diver- 
gent lamina of the occipital bone. It would require too long a notice, and also a 
demonstration of specimens, to render this subject fully evident; but the author is 
anxious to contribute, by a prompt correction of what he deems error, data to 
secure the fundamental basis of this important branch of anatomical study. 
{ 
On the Homologies and Notation of the Dental System in Mammalia. 
; i By Professor Owen, U..D., F.R.S. 
The Professor commenced by observing that one of the results of the determina- 
tion of the homologies of parts of the animal body was the power of denoting them 
__ by symbols, and gave, in illustration of the advantages of this substitute for verbal 
definitions, some descriptions of the order of development and change of dentition in 
different mammalia, and especially in the genus Macropus, Shaw. He had shown 
that the formula which had been supposed by Cuvier to distinguish the small kan- 
_ garoos (Halmaturus) from the Macropus of Cuvier, was the same essentially in both 
_ genera, the differences depending only on the length of time during which certain 
teeth were retained. The true formula of the Macropodide was— 
3g LOO Lael coded 
"a ° ono Tay a 
The canines are never functionally developed, though minute germs occur in the 
upper jaw of some of the smaller species, and in the embryo state of all kangaroos. 
The author had not described the changes of the teeth or given the deciduous for- 
mula of the kangaroos in his ‘Odontography,’ nor had any additional information 
been given in later works. Mr. Waterhouse, in his ‘ Natural History of Mammalia,’ 
had confirmed the author’s determination of the permanent formula of the dentition 
of the Macropodide, and had abandoned the Cuvierian one. 
3—3 1-1 2—2 
_ The deciduous dentition of Macropus Major was itor c—— 
1—1 2-—2 
: | Sig ss 
first permanent incisors) ; the molars were d 3 and d4 (the milk-molars homologous 
“with those so numbered in the typical dentition of the horse, hog). The next stage 
|the kangaroo is the acquisition of 7 2 in the upper jaw (second permanent incisor), 
and of m1 (first permanent true molar) in both jaws, formulised by— 
= 12. The incisors were 71 
nes) are shed, and the dentition is ? 
2 
‘ . .3— _ 2= 
_ Atone year old the dentition was i, dm —, ns = 24. The next stage 
is the shedding of d3 and the appearance in place of m3. ‘Then d4 is shed and- 
succeeded by p 4,—the single premolar which displaces d4 vertically. Finally, 
