367 
ADOLPHI (2, p. 475) even goes so far as to conclude that the tenth 
is the ancestral sacral vertebra, and that during evolution the pelvis 
has been gradually shifting forward so that at the present day the 
ninth vertebra usually carries the pelvis. Such a case as this pro- 
pounded by AporLenı would be in a sense a “travelling” or “‘migration” 
of the pelvis from the tenth to the ninth vertebra, but in using the 
expressions there is a danger of its being understood to mean an 
actual movement of parts which has occurred during ontogeny (e. g. 
PARKER 33, p. 715—716, and Gapow 15, p. 18). The term “homeosis”, 
coined by Barrson (4, p. 111) to express such variation of relations, 
is therefore a welcome addition to our morphological vocabulary. 
As soon as we admit that, in Anura at least, any vertebra can 
become sacral, and that it only requires the stimulating presence of 
the iliac cartilages to induce an exaggerated development of the di- 
apophyses, all the mystery of abnormal sacra is dispelled, whether the 
abnormality is due to the asymmetry, or to the compound nature of 
the sacrum, or to a combination of these. The cases on record of 
abnormal sacra in Anura are numerous, but probably represent only a 
small proportion of those which have been observed, and a still smaller 
proportion of the cases which actually occur in nature. Simple un- 
symmetrical sacra, in which the diapophyses are single, but arise on 
the right and left sides from different vertebree (usually one from the 
ninth and the other from the eighth or tenth), are described by Lioyp 
More@an in Rana (30, p. 53) and by SAsserno (36, Fig. XIV), CAmE- 
RANO (8, Fig. 4) and Howes (23) in Bombinator. Bilaterally com- 
pound sacra are met with occasionally in Bombinator (CAMERANO 8, 
Fig. 3, and Sasserno 36, Fig. XI). Sacra unilaterally compound, i. e. 
in which two diapophyses reach the ilium on one side of the body 
and but a single one on the other, are most numerous of all. Such 
are recorded in Bombinator (GörtE 18, Fig. 346, SASSERNO 36, 
Figs. 7, 8, 9, 10, 12, 13), Bufo variabilis (ApotpHr 1, Fig. 4), 
Bufo cinereus (Apotpni 3, p. 136, Fig. 11), Rana temporaria 
(Howes 22, p. 278, Fig. 1) and Rana esculenta (Howes 24, p. 269, 
Fig. 2a and 2b). I do not regard as compound those sacra in which 
the centra 8 and 9 are fused, but in which the anterior pair of di- 
apophyses do not share in supporting the pelvis. This form of mon- 
strosity is not uncommon, and cases of it are recorded by Howes 
in Rana esculenta (24, p. 269, Fig. 1a), Aporpnı in Rana 
esculenta (2, p. 485—486 and Fig. 11), and Bennam in Bufo 
pantherinus and Rana mugiens (5, p. 480). 
KOELLIKER (26, p. 237) says that in embryos of Pipa the ventral 
cartilage of the urostyle does not extend forward in front of the 
