Ma_y I, 1879] 



NATURE 



23 



clearly indicated by Gegenbaur,i ^jig points out that in mam- 

 mals, in contradistinction from reptiles, " the longitudinal axis 

 of the ilium gradually acquires an oblique direction, from in 

 front and above, back\saras 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 aud downwards ; 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 pro- 

 duce it in birds ; for, in the former, the ischium follows the 

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

 uothing 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 ihum of mammals answers to 

 the postacetabular part of the ilium of bir^is, and that "the 

 crista ossis Hit of mammals corresponds with the posterior edge 

 of the post-acetabular part of the bird's ilium. Between the two 

 parts, therefore, 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°. I cannot reconcile the contrary view either with 

 the relations of the ilium to the sacrimi, or with the attachment 

 of the muscles. 



On comparing the pelvis of Ornithorhynchus with that of a 

 lizard, or that of a chelonian, it will be observed that the resem- 

 blance between the former and the sauropsidan pelvis is, in most 

 respects, closer than that which it bears to the higher mammalian 

 pelvis. In the reptiles both the pubes and the ischia unite in a 

 ventral symphysis ; the pubis has a strong pectineal process, 

 which acquires very large dimensions in the Chelonia ; the 

 metischial processes are also often very strong. Nevertheless, 

 there is an important difference, for in all these animals the iliac 

 axis is either nearly perpendicular to the sacral axis, or slopes 

 from above downwards and forwards ; the obturator axis also 

 inclines downwards and forwards. Hence in most Lacertilia and 

 Chelonia, the pubes slope forwards very obliquely, while the 

 ischia come more and more forwards. 



In other words, such modifications of the pelvis as occur in the 

 Lacertilia and the Chelonia are of an opposite kind to those 

 which take place in mammalia. 



The same thing is true of the Crocodilia, 



Thus it appears to be useless to attempt to seek among any known 

 Sauropsida for the kind of pelvis w hich analogy leads us to expect 

 among those vertebrated animals which immediately preceded 

 the lowest known mammalia. For, if we prolong the series of 

 observed modifications of the pelvis in this group backwards, the 

 " pro-mammalia " antecedent to the Monotremes may be expected 

 to have the iliac and obturator axis perpendicular to the sacral 

 axis, and the iliopectineal axis parallel with it ; something, in 

 short, between ihe pelvis of an Ornithorhynchus and that of a 

 land-tortoise ; and provided, like the former, with large epipubes 

 intermediate in character between those of the lower mammals 

 and those of crocodiles. In fact, we are led to the construction 

 , of a common type of pelvis, whence all the modifications known 

 to occur in the Sauropsida and in the mammalia may have 

 diverged. 



It is a well-known peculiarity of the urodele amphibia, that 

 each OS innominatum consists of a continuous cartilage, the 

 Tentral half of which is perforated by a foramen for the obtu- 

 rator nerve, but has no large fibrous fontanelle or obturator 

 foramen in the ordinary sense of the word. As the junction of 

 the dorsal with the ventral moiety, the acetabulum marks off the 

 iliac portion of the pelvic arch above, from the pubic and ischial 

 regions below ; and these are further distinguishable, even apart 

 from their ossifications, by the position of the foramen for the 

 obturator nerve and the origins of the muscles. In full-grown 

 Ipecimens of Salamandra maculosa the pelvis presents the fol- 

 lowing characters : — The iliac axis is slightly inclined forwards, 

 while the iliopectineal axis is practically parallel with the sacral 

 axis. The iliac ossification extends into the acetabulum, and 

 forms a triangular segment of its roof with the apex downwards, 

 exactly as in lizards. The posterior and inferior side of the 



' "Bcilrige 2ur Kenutniss de; Beckens der VOgel," Jenaisclu Ztit- 

 Khrtfi vi. 



triangle is separated by a thin band of the primitive cartilage 

 from the upper edge of the similarly triangular cotyloid end of 

 the ischial ossification, the anterior edge of which is vertical 

 again as in lizards. Between this edge and the anterior and in- 

 ferior edge of the iliac ossification there is a cartilaginous inter- 

 space,- as in crocodiles, which represents the cotyloid end of the 

 pubis. This cartilaginous part of the pubis gives rise to a pec- 

 tineal process, which has the same position as in birds and in 

 Ornithorhynchtts, In the floor of the acetabulum the pubic ossi- 

 fication makes its appearance as a very thin lamina, which 

 extends, underneath the pectineal process, inwards ; and gradu- 

 ally surrounds the whole of the thickened transverse ridge of 

 cartilage which corresponds with the pubis. The pubis is thus 

 rei^resented by an axis of cartilage surrounded by bone, and the 

 thick inner extremities of the two pubes are largely united by 

 fibrous tissue. The ischia are relatively lar^e, and are united, 

 partly by cartilage and partly by ligament, in a long symphysis. 

 Their posterior and external angles are produced into short 

 meti^chial processes. In one specimen I observed a distinct 

 sutural line between the anterior curved edge of the right 

 ischium and the corresponding pubis, while no such suture could 

 be traced upon the other side. 



The pelvic arch of Salamandra, therefore, contains all the 

 elements which are found in the higher vertebrata, but the 

 obturator fontanelle is wanting, and it seems to me that in such 

 a pelvis we have an adequate representation of the type from 

 which all the different modifications which we find in the higher 

 vertebrata may have taken their origin. 



In the lizards and the Chelonia the iliac and obturator axes 

 have inclined forwards, and the epipubes have been reduced to 

 such rudiments, as have been described in chameleons and in 

 some tortoises.^ 



In the crocodiles, with the same general pelvic characters, the 

 cotyloid end of the pubis retains its imperfectly ossified condi- 

 tion, while the epipubes represent the vastly enlarged rami of the 

 salamandrine epipubis. 



In the Ornithoscelida and in birds, the ilia elongate, but it is 

 the modification of the pubes and ischia which is the most cha- 

 racteristic feature of the pelvis, and the epipubis vanishes. 



In the Pterosauria and in the Dicynodonts, the salamandrine 

 non-development of an obturator fontanelle persists ; and, in the 

 former, the sessile rami of the epipubis appear to be represented 

 by the so-called marsupial bones. 



Unless the like should prove to be the case in the Dicynodonts, 

 it is in the mammalia alone that the subsacral portion of the 

 ilium elongates backwards, carrying with it the pubis and the 

 ischium, between which a large rounded obturator fontanelle is 

 developed. 



These facts appear to me to point to the conclusion that the 

 mammalia have been connected with the amphibia by some 

 unknown pro-mammahan group, and not by any of the known 

 f i;rms of Sauropsida ; and there is other evidence which tends 

 in the same direction. 



Thus, the amphibia are the only air-breathing vertebrata 

 which, like mammals, have a dicondylian skull. It is only in 

 them that the articular element of the mandibular arch remains 

 cartilaginous ; while the quadrate ossification is small, and the 

 squamosal extends down over it to the osseous elements of the 

 mandible ; thus affording an easy transition to the mammalian 

 condition of these parts. 



The pectoral arch of the Monotremes is as much amphibian'as 

 it is sauropsidan ; the carpus and the tarsus of all Sauropsida, 

 except the Chelonia, are modified away from the urodele type, 

 while those of the mammal are directly reducible to it ; and it is 

 perhaps worth notice, that the calcar of the frogs is, in some 

 respects, comparable with the spur of the Monotremes. 



Finally, the fact that in all Sauropsida it is a right aortic arch 

 which is the main conduit of arterial blood leaving the heart, 

 while, in mammals it is a left aortic arch which performs this 

 office, is a great stumbling-block in the way of the derivation of 

 the mammaha from any of the Sauropsida. But if we suppose 

 the earliest forms of both the mammalia and the Sauropsida to 

 have had a common amphibian origin, there is no d fficulty in 

 the supposition that, from the first, it was a left aortic arch in 

 the one series, and the corresponding right aortic arch in the 

 other, which became the predominant feeder of the arterial 

 system. 



The discovery of the intermediate links between reptiha and 



■ Hoffman, " Beitrage zur Kenntniss des Beckens der Amptiibien und 

 Reptilien," Nied. Archiv/Hr Zoologie, 1876. 



