ON THE VERTEBRATE SKELETON. 285 



The scapulo-coracoid arch, both elements of which retain the form of 

 strong and thick vertebral and sternal ribs in the crocodile, is applied in the 

 skeleton of that animal over the anterior thoracic haemal arches. Viewed 

 as a more robust haemal arch, it is obviously out of place in reference to the 

 rest of its vertebral segment. If we seek to determine that segment by the 

 mode in which we restore to their centrums the less displaced neural arches 

 in the sacrum of the bird (fig. 27, n \-n 4), we proceed to examine the verte- 

 bra? before and behind the displaced arch with the view to discover the one 

 which needs it in order to be made typically complete. Finding no centrum and 

 neural arch without its pleurapophyses from the scapula to the pelvis, we give 

 up our search in that direction ; and in the opposite direction we find no verte- 

 bra without its ribs until we reach the occiput : there we have centrum and 

 neural arch, with coalesced parapophyses — the elements answering to those 

 included in the arch N i, fig. 5— but without the arch Hi; which arch 

 can only be supplied, without destroying the typical completeness of antece- 

 dent cranial segments, by a restoration of the bones 50-52, to the place which 

 they naturally occupy in the skeleton of the fish. And since anatomists 

 are generally agreed to regard the bones 50-52 in the crocodile (fig. 22) 

 as specially homologous with those so numbered in the fish (fig. 5), we 

 must conclude that they are likewise homologous in a higher sense ; that in 

 fig. 5 the scapulo-coracoid arch is in its natural or typical place, whereas in 

 the crocodile it has been displaced for a special purpose. Thus, agreeably 

 with a general principle, we perceive that as the lower vertebrate animal 

 illustrates the closer adhesion to the archetype by the natural articulation of 

 the scapulo-coracoid arch to the occiput, so the higher vertebrate manifests 

 the superior influence of the antagonising power of adaptive modification by 

 the removal of that arch from its proper segment. 



The scapula retains the more common cylindrical long and slender rib- 

 like form of the pleurapophysis in the chelonian reptiles, where, from the 

 greater length of the neck, it has retrograded further than in the crocodile 

 from its proper centrum, and is placed not upon, but within, an anterior 

 thoracic haemal arch, the pleurapophysis of which has, on the other hand, 

 been expanded like a scapula. 



If the arguments founded upon the relations of the scapulo-coracoid arch 

 to the segments of the skeleton in osseous fishes and crocodilians be admitted 

 to sustain the conclusion here drawn from them, that arch must be held to 

 form the haemal complement of the occipital vertebra in all animals. Bojanus, 

 in illustrating his vertebral theory of the skull by the osteology of the Emys 

 Europcea, thus defines the 



"Vertebra occipitalis, sive capitis prima. 



" Basis occipitis, seu corpus hujus vertebrae, 



" Pars lateralis occipitis, sive arcus, 



" Crista occipitalis, processus spinosi loco, 



" Cornu majus hyoidis, costce vertebrae occipitalis comparandum *." 



He adds a dotted outline of the hyoid arch to complete the vertebra oc- 

 cipitalis, in tab. xii. fig. 32, B. 1 of his beautiful Monograph. 



Supposing the special homology of the middle cornua of the hyoid of the 

 chelonian, so represented and compared to ribs by Bojanus, with the stylo-, 

 epi- and cerato-hyals of the fish (fig. 5, 38, 39, 40) to have been correct, which 

 the metamorphoses of the hyoid and branchial arches in the batrachians dis- 

 prove, the singular and highly interesting change of position as well as shape 

 of the true ceratohyals, during the same metamorphosis, prepares us to expect 

 a retrogradation of the hyoid arch in respect to its proper centrum, in the 

 * .Anatome Testudinis Europaeae, fol. 1819, p. 44. 



