396 



Prof. Huxley on the 



[Mar. 6, 



Of these I shall term a longitudinal line traversing the centre of the 

 sacral vertebrse, the sacral axis (Plate 8, 8. a.) ; a second, drawn along the 

 ilium, dorso-ventrally, through the middle of the sacral articulation and 

 the centre of the acetabulum, will be termed the iliac axis (11. a.) • a 

 third, passing through the junctions of the pubis and ischium above and 

 below the obturator foramen, will be the obturator axis (Ob. a.) ; while a 

 fourth, traversing the union of the ilium, in front with the pubis, and 

 behind with the ischium, will be the iliopectineal axis (Tp. a.). 



The least modified form of mammalian pelvis is to be seen, as might 

 be expected, in the Monotremes, but there is a great difference between 

 Ornithorhynchus and Echidna in this respect, the former being much 

 less characteristically mammalian than the latter. 



In Ornithorhynchus (Plate 8, fig. 4), the ilium is remarkably narrow, 

 and the angle between the iliac and the sacral axis is large, so that the 

 ilium is but very slightly inclined backwards. The iliopectineal axis, 

 nearly at right angles with the iliac axis, is inclined to the sacral axis 

 at an acute angle ; while the obturator axis is nearly perpendicular to the 

 sacral axis, and the obturator foramen is relatively small. The front 

 margin of the cotyloid end of the pelvis sends off a very strong 'pec- 

 tineal process (p.p.), from the inferior basal part of which a short, obtuse 

 tub erculum pubis (t. p.) projects. Between this and the symphysis, the 

 base of the marsupial bone (Ep.p.) is attached. The ventral rami of the 

 pubes are short and, like those of the ischium, they are united through- 

 out their whole length in a long symphysis, the ischial division of 

 which (Sy. J.) is as long as, if not longer than, the pubic division 

 (Sy. p.). The cotyloid ramus of each ischium gives off a stout elongated. 

 metischial process (m.p.) backwards. 



In Echidna (Plate 8, fig. 5), on the other hand, the ilium is much 

 broader ; while the iliac axis inclines downwards and backwards, at an 

 acute angle with the sacral axis. The iliopectineal axis being still at 

 right angles with the iliac axis, makes a much larger angle with the 

 sacral axis ; and the obturator axis is inclined from above, at an angle 

 of nearly 45° to the sacral axis, downwards and backwards. In fact, the 

 change in the general character of the pelvis seems to result from its 

 ventral elements having been carried backwards and upwards by the 

 backward and upward shifting of that portion of the ilium which lies 

 below the le^el of its articulation with the sacrum. There are other 

 changes by which the aspect of the pelvis is much altered. The inner 

 wall of the acetabulum is incompletely ossified, but, in other respects, 

 the pelvis makes a considerable approximation towards the ordinary 

 mammalian form. Thus the pectineal process is represented by a less 

 prominent and more elongated ridge ; the metischial process widens 

 out into a mere triangular expansion or "tuberosity," of the ischium, 

 and the symphysial union of the ischia is short. 



Tn all other Mammalia (e.g., Lepus, Plate 8, fig. 6) the iliac axis forms 



