1879.] Characters of the Pelvis in the Mammalia, Sfc. 397 



as acute, if not a more acute, angle with the sacral axis ; the angle be- 

 tween the ilio pectineal axis and the sacral axis more and more approaches 

 a right angle ; and that between the sacral axis and the obturator axis 

 becomes more and more acute. The obturator foramen acquires a much 

 larger proportional size. The symphysial union becomes restricted to a 

 greater or less portion of the pubes ; or the ventral halves of the 

 ossa innominata may cease to be directly united, even the pubes being 

 far apart in the dry skeleton. The metischial processes are represented 

 by tuberosities, which may extend upwards and unite with anterior 

 caudal vertebras ; and the ilia may remain narrow or become 

 extremely expanded. In all monodelphous Mammalia the marsupial 

 bones disappear. 



The distinctive features of the mammalian pelvis have been clearly 

 indicated by Gregenbaur,* who points out that in mammals, in contra- 

 distinction from reptiles, " the longitudinal axis of the ilium 

 gradually acquires an oblique direction, from in front and above, 

 backwards and downwards. The part which represents the crista 

 above thus becomes turned forwards, or more or less outwards, with 

 increase of lateral surface ; the acetabular part backwards and down- 

 wards ; hence the ischium retains its original direction in the 

 produced long axis of the ilium and, at the same time, takes up a 

 position in relation to the vertebral column similar to that which 

 obtains in birds. The conditions of this position are, however, to be 

 sought in factors of a totally different nature in mammals from 

 those which produce it in birds ; for, in the former, the ischium 

 follows the changed direction of the ilium, whilst in birds, the ilium 

 has nothing to do with the matter, and the ventral elements of the 

 pelvis appear to pass towards the caudal region, independently of the 

 ilium." 



On one point, however, I cannot agree with Gegenbaur's con- 

 clusions. He is of opinion that the ilium of mammals answers to 

 the post-acetabular part of the ilium of birds, and that " the crista 

 ossis ilii of mammals corresponds with the posterior edge of the post- 

 acetabular part of the bird's ilium. Between the two parts, there- 

 fore, there is the difference of a rotation through an angle of almost 

 180°." On the contrary, it appears to me evident that the whole 

 crista ilii in a mammal corresponds with the whole dorsal edge of the 

 ilium in a bird or a reptile, and that the angle through which the 

 iliac axis rotates amounts to not more than 90° (compare Plate 8, 

 fig. 6, Lepus, with fig. 9, Apteryx). I cannot reconcile the contrary 

 view either with the relations of the ilium to the sacrum, or with the 

 attachment of the muscles. 



On comparing the pelvis of Ornithorhynckus with that of a lizard 



* "Beitrage zur Kenntniss des Beckens der Vogel," " Jenaisclie Zeitschrift," vi. 



