328 report— 1846. 



The special homologies of these elements of the pelvis being thus deter- 

 mined, it follows, that their general homology, as it may be revealed by the 

 simple condition of the pelvic arch in the species in which the pelvis, as 

 complete and fixed to a sacrum, makes its first appearance in the animal 

 kingdom, will be equally applicable to the parts under all their metamor- 

 phoses in the higher air-breathing vertebrates. 



The correspondence of the segment of the endoskeleton in the menopome 

 D', PI', H, A, with the typical vertebra, as illustrated by fig. 15, is such, 

 that any other explanation of its essential nature than as a representative or 

 repetition of such fully developed segment or vertebra seems contrary to 

 nature. The chief modification has its seat in the most peripheral part or 

 appendage A, as compared with its simple homologue in the thoracic segment 

 of the bird (fig. 15). If 62 and 64 are to be regarded as strangers to the 

 vertebral system, new parts introduced for special purposes, and not as 

 normal elements modified for special purposes, I am at a loss to know on 

 what principles, or by what series of comparisons we can ever hope to attain 

 to the higher generalizations of anatomy, or discover the pattern according 

 to which the vertebrate forms' have been constructed. It may be said that the 

 arch which they constitute performs a new function, inasmuch as it sustains 

 a locomotive limb which reacts upon the ground. But this new function 

 arises in the menopome, rather out of the modifications of the appendage 

 than of the arch itself. In so far as the mere support of the appendage is 

 concerned, the inverted or haemal arch PI', H, performs no new function, but 

 one which is common to such arches in the thorax of birds, and to the less com- 

 pletely ossified homologous arches in the abdomen of fishes, where moreover 

 the simple diverging appendages do give attachment to the muscles of locomo- 

 tion. Comparing, then, the haemal arch in question with that of the typical 

 vertebra (fig. 15), we find that, like the scapulo-coracoid arch in fishes 

 (fig. 5, H i), its parts are open to two interpretations. The upper piece of 

 PV may be thewhole pleurapophysis, the lower, 62, the haemapophysis, and the 

 part, 64, the half of an expanded and bifid haemal spine : or PV with 02, may 

 be two portions of a ideologically compound pleurapophysis, and 64 the haem- 

 apophysis, which would join with its fellow without, or with a mere rudiment 

 of, a haemal spine intervening. From the analogy of the scapulo-coracoid 

 arch in fishes, which is proved by its modifications in higher animals to 

 want the haemal spine, it is most probable that such is the condition and 

 true interpretation of the correspondingly simple pelvic arch under considera- 

 tion. But the general relation of this arch to the haemal one of the typical 

 segment is not affected by the alternative. 



I regard, therefore, PI 1 , 62, as two portions of a fully developed pleurapophy- 

 sis ; and the pleurapophyses, PI', PI of the contiguous vertebrae as answering 

 only to the upper portion of the pelvic one. In ascending from the meno- 

 pome to the crocodile, we find the homologue of 62 broader than it is long, 

 and articulated to the thickened proximal portions of the pleurapophyses of 

 two segments ; and we observe, likewise, the pelvic arch completed below 

 by two pairs of haemapophyses : for the anterior pair the name of ' ossa 

 pubis ' is retained ; to the posterior pair that of ' ischia' is given. In general 

 homology these bones complete, as haemapophyses, the two vertebral seg- 

 ments modified to form the sacrum of the crocodile ; and the intermediate 

 connecting piece (ilium) may be interpreted, as either the confluent distal 

 portions of the pleurapophyses of both vertebrae, or as an expansion of one 

 such portion, answering to 62 in the menopome, and intruding itself between 

 the stunted pleurapophysis and distant haemapophysis of the second sacral 

 vertebrae in the crocodile. 



