860 SELLE TON 
much aborted digit seems in embryonic Ostriches to bear a CLAW 
(page 89). In the Sphenisct the pollex is more or less completely 
fused with the index, which latter is made up of two long pha- 
langes, while the third digit consists of one long phalanx. Casuarius, 
Dromxus and Apteryx retain only the index, the first and third 
digits being either lost or reduced to insignificant traces.1 
THE PELVIC ARCH is the portion of the PELVIS (page 703) 
which is made up of the ILIUM (page 458), ISCHIUM (page 460) and 
0s PUBIS (page 748), the last three being paired bones which meet 
on each side at the acetabulum or cup that receives the head of the 
FEMUR (page 248), coalescing with each other at an early stage. 
The ilium may be conveniently divided into a preacetabular and 
a postacetabular part, the relative proportions of which afford some 
useful characters ; thus in the Gallinw they are nearly equal, but 
Common Fowt, Embryo and Chick. 
Ac, Acetabulum ; Fis, Foramen ischiadicum ; Pr.p. Processus pectinealis. 
in the Accipitres the anterior is longest, while in Colymbus, where 
most of the pelvis is drawn out backward, it is only half the length 
of the posterior. ‘To the inner surface of the ilium, and in most 
cases near its ventral margin, are attached the lateral processes of 
the cruro-sacral vertebrae (page 855), while near its dorsal rim is 
attached all the rest of the sacrum. The outer surface of the 
preacetabular portion forms a broad vertical blade with a more or 
less deep concavity which serves for the origin of the external 
iliac muscles (often mistakenly called gluteal). In rare cases the 
right and left preeacetabular portions fuse with each other along 
their dorsal edge above the spinous processes of the neighbour- 
ing vertebre and enclose a canal on each side. In front of the 
1 As an appendix to the account of this part of the Skeleton, mention should 
here be made of the Sesamoid Bones. These are not disconnected portions of 
the framework, but cartilaginous or osseous formations either within the 
capsules of joints, as the hwmero-scapular bone in the capsule of the joint of 
that name, or within the inserting tendons of muscles, as the patella of the 
knee and the patella ulnaris of the Sphenisci, and osseous or cartilaginous nodules 
in the tendons of the long muscles of the toes, of the muse. extensor metacarpt 
radialis and of the muse. propatagialis. 
