Karl Pearson and Adelaide G. Davin 
307 
The following rules appear to us to hold for the somewhat heterogeneous material 
classed as Edentata. 
(i) Lunulae may occur. 
(ii) There are no cases at all of the mesial fahella (with or without the 
lateral fahella) occurring*. 
(iii) When the cyainella occurs it is very extensive and when it occurs there 
is no lateral fahella. 
(iv) Conversely when the lateral fahella occurs there is no cyamella. 
(v) There are cases in which no sesamoids whatever of the knee-joint have 
been found. 
We have accordingly reached a stage in which the mesial fahella has not been 
evolved, for it does not seem to exist in any earlier types. Further we have 
reached a stage where a non-sesamoidal type of Monodelphia — a type afterwards 
so widely developed in the Ungulata — is seen alongside a sesamoidal type after- 
wards so widely developed in Rodentia, Insectivora, Primata and Garnivora. It 
would seem as if the Edentata as modern representatives of these primitive 
Monodelphia must be from the knee-joint sesamoidal standpoint most suggestive 
from their very heterogeneity. Meckel speaks of the external fahella only as 
existing in the Edentates, and cites as examples the Anteater and Sloth (Ai). He 
does, however, say that this external /aftei/a "indessen mehr dem Kneekehlmuskel 
gehortf." He also found this sesamoid very large in the Edentates, much larger 
than in other animals, a remark we can fully confirm. 
GruberJ found in Bradypus tridactylus (the three-toed sloth) and in Dasypus 
seocoinctns (six-banded armadillo) no sesamoids in the heads of gastrocnemius or in 
the tendon of poplite us, but in Myrmecophaga there was in the tendon of popliteus 
a sesamoid articulating with the cajntul urn fibulae. 
Gruber's result for Bradypus is somewhat surprising, for while there are no 
fahellae there exists, as Blainville first noted, a great cyamella; there is in addition 
a lunula. The point is well put by Mackintosh in his paper " On the Myology of 
the Genus Bradypus"^ that while there are no fahellae in gastrocnemius : " There 
is a large wedge-shaped fahella [i.e. cyamella] in the origin of this muscle 
[i.e. popliteus], the tibial surface of which is the larger and articulates both with 
the femur and the tibia." 
We are, we think, reaching a stage of evolution in which lateral fahella and 
cyamella have coalesced and form a single large sesamoid. This large sesamoid 
may articulate with both femur and tibia as in Bradypus, or form a link between 
the femur and the capitidum fibulae as in the Anteater|j. Gruber, who never 
* Compare what is stated later as to Onjcteropiis, p. 369. 
t Loc. cit. p. 635. + Loc. cit. p. 62. 
§ Proc. R. Irish Acad. Vol. i. Science, 2nd Series, p. 528. 
II This alternative, tibio-femoral or fibulo-femoral, link of the compound sesamoid might con- 
ceivably have bearing on the appearance of fahellae in bot h heads of gastrocnemius. 
