22 FOSSIL REPTILIA OF THE LONDON CLAY. 



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 anthropotomist, by his mode of counting and defining the dorsal vertebrae and 

 ribs, admits, unconsciously perhaps, the important principle in general homology 

 which is here exemplified, and which, pursued to its legitimate consequences and 

 further applied, demonstrates that the scapula is the modified rib of that centrum and 

 neural arch which he calls the ( occipital bone,' and that the change of place which 

 chiefly masks that relation (for a very elementary acquaintance with comparative 

 anatomy shows how little mere form and proportion affect the homological characters 

 of bones) differs only in extent and not in kind from the modification which makes a 

 minor amount of comparative observation requisite, in order to determine the relation 

 of the shifted dorsal rib to its proper centrum in the human skeleton. 



With reference, therefore, to the occipital vertebra of the crocodile, if the com- 

 paratively well- developed and permanently distinct ribs of all the cervical vertebras 

 prove the scapular arch to belong to none of those segments, and, if that haemal arch be 

 required to complete the occipital segment, which it actually does complete in fishes, then 

 the same conclusion must apply to the same arch in other animals, and we must regard 

 the occipital vertebra of the tortoise as completed below by its scapulo-coracoid arch 

 and not, as Bojanus supposed, by its hyoidean arch.* 



Having thus endeavoured to show what the scapular arch of the crocodile is, I 

 proceed to point out the characteristic form of its chief elements. The upper and 

 principal part of the scapula (51, fig. 9) is flattened, and gradually becomes narrower to 

 the part called its neck, which is rounded, bent inwards, and then suddenly expanded to 

 form a rough articular surface for the coracoid, and a portion of a smoother surface for the 

 shoulder-joint. The contiguous end of the coracoid (52) presents a similar form, having 

 not only the rough surface for its junction with the scapula, but contributing, also, one 

 half of the cavity for the head of the humerus. It is perforated near the interspace 

 between these two surfaces. As it recedes from them, it contracts, then expands and 

 becomes flattened, terminating in a somewhat broader margin than the base of the 



* Anatome Testudinis Europaea. fol., 1S19, p. 44. Geoffroy St. Hilaire selected the opercular andsub- 

 opercular bones to form the inverted arch of his seventh (occipital) cranial vertebra, and took no account of 

 the instructive natural connexions and relative position of the hyoidean and scapular arches in fishes. With 

 regard to the scapular arch, he alludes to its articulation with the skull in the lowest of the vertebrate 

 classes as an ' anialgame inattendue' (Anatomie Philosophique, p. 481): and elsewhere describes it as a 

 " disposition veritablement trfes singuliere, et que le manque absolu du cou et une combinaison des pieces 

 du sternum avec celles de la tete pouvoieut seules rendre possible." — Annates du Museum, ix, p. 361. A due 

 appreciation of the law of vegetative uniformity or repetition, and of the ratio of its prevalence and power to 

 the grade of organization of the species, was, perhaps, essential in order to discern the true signification of 

 the connexion of the scapular arch in fishes. 



