Karl Pearson and Adelaide G. Davin 
371 
The typical Edentate may be said to be characterised by a large cyameUa with 
probably one or more lunulae. The mesial fahella has practically no existence for 
this order, and in the few cases where the lateral fahella appears, it seems inter- 
changeable with the cyamella. There is one group, however, Dasi/podidae in which 
neither cyamella nor fahellae appear. The order of Edentates therefore from our 
present standpoint contains two fundamental groups one of which consists of species 
with non-sesamoidal knee-joint and the other of species with a sesamoidal knee-joint. 
The characteristic of the latter is the absence of the mesial fahella and the co- 
alescence of lateral fahella and cyamella. to form a large sesamoid which may act as 
an articulating link between the femur and one or other of the lower members of 
the limb. 
(17) Mars upials. 
It will be clear to the reader that the views expressed above on the affinities of 
genera, and orders based on the changes in the sesamoids of the knee-joint must 
receive strong confirmation, or failing it the coup de grace, when we come to the 
Marsupials. 
Historically we find the same vagueness as to the knee-joint sesamoids as in the 
Edentates. Meckel talks about the sesamoid of the kangaroo as if the kangaroo 
presented an " external fabella " only, and he makes a similar statement as to the 
opossum. Before Meckel Blumenbach had already noticed the external sesamoid in 
the Didelplda generally. Other fairly early notices of the external sesamoid are 
Pandir and d' Alton in Didelphis cancrivora, Halmaturvs elegans and H. giganteus 
(Meckel also noticed it in Halmaturvs), Cuvier and Laurillard in Phalangista 
carifrons, and Owen in Myrmecohius and some other marsupials. Owen describes 
the sesamoid as above and behind the external condyle of the femur*. 
Gruber while still speaking of the lateral sesamoid of the Marsupials, as if it 
were directly comparable with the external /afteZZa in other orders, gives useful in- 
formation on its muscular relations. In Didelphis philander he tells us it is in the 
tendon of gastrocnemius externus, but articulates with the head of the fibula. The 
same statement applies to the sesamoid in Dasyurus viverrinus. Again in Flialan- 
gista vulpina, the external sesamoid articulates with the capituhun fihulae and is 
united to the fibula "durch eine schlaife Gelenkkapsel und durch ein starkes 
Ligament." It is " fiddle-shaped " and with its thicker upper end the tendon of 
origin of gastrocnemius externus is united (" verwachsen "). In these three genera 
the external ossicle has no relation to the knee capsule and popliteus does not spring 
from the external condyle of the femur, but from the knee capsule and the capitulum 
fihulae'f. None of the above has any internal fabella. These three species give very 
typical marsupial sesamoidal systems. They are cases really in which fahella and 
cyamella have coalesced and we do not believe it possible to speak of merely the 
" lateral " fahella existing. The remaining two cases, if Gruber has interpreted 
them rightly, seem to us more or less aberrant variants of the main marsupial type. 
They are : 
* Anatomy of Vertebrates, Vol. ii. p. 358. t Loc. eit. p. 59. 
