Karl Pearson and Adelaide G. Davin 
378 
Plate XXIX, Fig. 79, gives a second wombat; here the big sesamoid is not fused 
to the head of the fibula, but it nevertheless consists of the associated lateral 
fahella and the cyamella fused together, a true jjarafibula. The limb has had to be 
extended to separate the sesamoid from articulating on the femur. The reader 
should now examine our drawing after Pfitznur in Plate V, Fig. 13, of this memoir, 
where the muscular attachments have been entirely removed; he will (as a careful 
study will show him in the present plates) be able to detect the two portions of the 
associated bone, which become respectively the lateral /a6e//« and cyamella. These 
two portions of the great marsupial sesamoid of the knee-joint are also quite dis- 
tinguishable in Fhalangista. See our Plate XXX, Fig. 80. 
While the hypothesis that lateral /aZ)e/^a and cyamella are the product of the 
breakdown of the parafibula finds much support from the fact that specimens occur 
in which these sesamoids are actually fused together and to the fibula*, we have to 
consider how far this hypothesis fits the myological facts and tends to throw light 
on Gruber's description of the lateral /«6e//rt in man as a muscular " Knotenpunkt." 
We have above all to ask whether the Marsupials themselves provide evidence in 
favour of this break-up of a parafibula. One of the most noteworthy facts in this 
respect is the occurrence in certain marsupials of a hemisesamoidal cyamella, this 
articulates on the head of the fibula, and upon this cyamella articulates the lateral 
fahella, which in its turn is in contact with the upper part of the condyle of the femur. 
A specimen of Macropus (sp. ?) sent to us from Australia shows a lateral /a6e^/« 
— an orthosesamoid — and a hemisesamoid cyamella. The specimen consisted only 
of the lateral branch of the gastrocnemius with its orthosesamoid, the hemisesa- 
moidal cyamella and the external semilunar cartilage all attached, these for the 
left hind limb. It was thus not possible to draw the knee-joint showing these in 
their natural position. It is not, however, difficult to understand their relations to 
other structures in the knee-joint, if a comparison be made between these drawings 
and tho.se of other members of the Marsupialia. See Plate XXXIII, Figs. S8 n 
and 83 b. 
In a museum sj^ecimen of Macropus agilis we found also this hemisesamoidal 
cyamella, while the lateral fabella was a large orthosesamoid. In the mesial head 
of gastrocnemius there was a cartilaginous nodule, possibly a hemisesamoid. 
In another mounted specimen of Macropus rufus the large lateral sesamoid was 
simply wired on to the femur, a process which completely fails to indicate the sesa- 
moid system as a link between femur and capitulum fibulae. Our Plate XXXIV, 
Figs. 87 and 88, shows museum specimens of Macropus hennettii and Perameles 
lagotis in which this sesamoid is simply pinned on to the femurf . Our Plate XXXV, 
Fig. 90, gives what appears to us to be the position of this sesamoid in Bennett's Wal- 
laby ; it has very clearly no relation to the position as indicated in Figs. 87 and 88. 
* We have uever in examining several hundred knee-joints found a case in which the sesamoids were 
fused to the femur, and the only recorded case of ankylosis that we have met with is that reported by 
Gruber in a specimen of Myogale. 
t We have also found the parafibular sesamoid treated in this manner in the case of Dcndrohirjiis 
ursinus, where the sesamoid is generally similar to that of Macropus betuwttii, and again in Macrnpii.^i major. 
Biometrika xiii 24 
