Karl Pearson and Adelaide G. Davin 
359 
squirrel), Basyprocta aguti (golden agouti)*, Gavia porcellus (guinea-pig), Mijo- 
potamus coypus (coypu), in a second specimen only the external fahella was pre- 
served, Anomalurus fraseri , in this case the cyamella had been preserved. 
In a skeleton of Dolichotis patachonica (Patagonian cavy) no fabellae remained, 
but there wei-e exceedingly well-marked depressions for the fabellae on the 
condylesf, and in another skeleton of Dasyprocta aguti where the two fabellae had 
disappeared their positions were well marked by depressions on the condyles. 
There were well developed pits also on the condyles, almost certainly for fabellae, in 
Hystrix leucura. Occasionally when the fabellae were not traceable, lunulae had 
been preserved; thus in Hydromys chrysogaster (golden-bellied rat) we found both 
anterior lunulae. In another specimen, however, we found both fabellae and the 
anterior lunulae in situ. These are reproduced in Plate XXIII, Fig. 57 a. The lunulae 
of the anterior border of the semilunar cartilages existed in another skeleton of 
Dolichotis patachonica'f. In a mounted specimen of Hystrix cristata (crested 
porcupine) the fabellae and cyamella failed but there appeared to be an external 
anterior lunula, while in Synetheres insidiosus (a South American poi'cupine) there 
were, if there ever had been any, no remaining sesamoids, not even the lunulae. 
Parsons, who has dealt at length with the myology of the rodents, wrote as 
follows in 1894| of the Hystriomorphinae : 
The presence or absence of the fabellae does not seem to depend on the affinities of the animal, 
as they are large in A ulacodus on both sides, while in Myopotamm, only the outer one is present. 
In Dasyprocta they ai-e both present, in Calogenys both absent. 
We have found in a specimen of Myopotamus both fabellae present. 
Again : 
In the Sciuroiiiorpki?tae, however, they were Anuid in every case except that of Castor 
canadiensis {Sciurus, Pteromys, Xerus, Spermophilus, Arctomys, Castor). 
In a paper of 1896§ on the "Myology of the Rodents," Parsons writes that all 
except fam. Spalacidae {Rhizomys, Georychus, Bathyergus) have two fabellae. 
It will be seen that a good deal of work remains to be done on the rodents. It 
is not easy to obtain adequate material to determine an isolated problem of the knee- 
joint like the present. It is to be hoped that myologists in the future may pay atten- 
tion to the existence of cyamella and lunulae as well as of the fabellae. But our 
experience shows us that without a most careful examination, which in many cases 
ought to be microscopic, it is by no means easy to ascertain the presence or absence 
of these sesamoids even in most specimens of the smaller rodents. Further, it is 
quite clear from the material we have gathered together that persistent presence 
or absence in the same species cannot be asserted from the examination of isolated 
specimens. 
* Windle found hoth. fabellae in Dasyprocta isthmica, Journal of Anatonuj and Physioloriy, 1896 — 97, 
p. 352, and Mivart and Murie, Proc. Zool. Society, 1866, found both in Dafyprocta cristata. 
t Windle, Journal of Anatomy and Physioloyy , 1896 — 1897, p. 3.52, says he found only the lateral 
fahella. 
X "Myology of the Sciuromorphinae and Hystriomorphinae," Proc. Zool. Society, 1894, p. 291. 
§ Proc. Zool. Society, 1896, p. 181. 
23—2 
