96 BRITISH FOSSIL REPTILES. 



median portion above specified retains most of tlie formal characters of the centrum, 

 but there is a pair of long, slender, symmetrical ossicles, which, from the seat of their 

 original development, and their relative position to the neural arch, must be regarded 

 as also parts of its centrum. And this ossification of the element in question from 

 different centres will be no new or strange character to those who recollect that the 

 vertebral body in man and mammalia is developed from three centres. The term 

 'vomer' is applied to the pair of bones 13, in fig. 12, because their special homology 

 with the single median bone, so called in fishes and mammals, is indisputable; but a 

 portion of the same element of the skull retains its single symmetrical character in the 

 Crocodile, and is connate with the enormous pterygoids (24), between which it is 

 wedged. In some Alligators {All. niger) the divided anterior vomer extends far 

 forwards, expands anteriorly, and appears upon the bony palate. 



Almost all the other bones of the head of the Crocodile are adjusted so as to 

 constitute four inverted arches, respectively completed or closed below at the points 

 marked H iv, H iii, H ii, and H i, in fig. 13. These are the haemal arches of the 

 four segments or vertebrae, of which the neural arches have been just described. But 

 they have been the seat of much greater modifications, by which they are made sub- 

 servient to a variety of functions unknown in the haemal arches of the rest of the body. 

 Thus the two anterior haemal arches of the head perform the office of seizing and 

 bruising the food ; are armed for that purpose with teeth : and, whilst one arch is 

 firmly fixed, the other works upon it like the hammer upon the anvil. The elements 

 of the fixed arch (H iv), called ' maxillary arch,' have accordingly undergone the 

 greatest amount of morphological change in order to adapt that arch to its share in 

 mastication, as well as for forming part of the passage for the respiratory medium, which 

 is perpetually traversing this haemal canal in its way to purify the blood. Almost the 

 whole of the upper surface of the maxillary arch is firmly united to contiguous parts of 

 the skuU by rough or sutural surfaces, and its strength is increased by bony appendages, 

 which diverge from it to abut against other parts of the skull. Comparative anatomy 

 teaches that, of the numerous places of attachment, the one which connects the 

 maxillary arch by its element (20) with the centrum (13) and the descending plates of 

 the neurapophyses (14) of the nasal segment, is the normal or the most constant point of 

 its suspension, the bone (20) being the pleurapophysial element of the maxillary arch : 

 it is called the 'palatine,' because the under surface, shown in PI. A, 2, and PI. 1 B, 

 at 20, forms a portion of the bony roof of the mouth called the ' palate.' 



It is articulated at its fore part with the bone (21) in the same plates, which bone 

 is the haemapophysial element of the maxillary arch. It is called the ' maxillary,' and 

 is greatly developed both in length and breadth ; it is connected not only with 20 

 behind, and 22 in front, which are pai'ts of the same arch (see fig. 13), and with the 

 diverging appendages of the arch, viz. (26) the malar bone, and (24) the pterygoid, 

 but also with the nasals (15) and the lachrymal (ic), as well as with its fellow of the 



