ON THE VERTEBRATE SKELETON. 293 



which may be divaricated like the halves of the mandibular arch, so, as to 

 widen] the mouth laterally ; and this free suspension and incomplete closure 

 of the principal costal arches of the cranium in serpents repeats in an inter- 

 esting manner the characteristic free and open condition of all the costal arches 

 of their trunk. In the genus Tythlops the diverging appendage of the 

 palato-maxillary arch is reduced to the primitive condition of a long and 

 slender ray. In anourous batrachians a long and slender backwardly pro- 

 duced exogenous process of the haemapophysis (maxillary) joins a shorter 

 advancing exogenous process of the distal division of the next pleurapo- 

 physis (tympanic) : but in the tailed species the maxillary arch is fixed only 

 by a broad (pterygoid) appendage; and both maxillary and premaxillary retain 

 only their essential connections as forming the inferior arch of their segment. 

 In the proteus and siren the pleurapophysis (maxillary) is almost obsolete. 



The bones nos. 24, 24', 26 and 27, being shown to be the least constant 

 members of the group forming the nasal segment, and to form by their posi- 

 tion and direction, the diverging appendages of the haemal arch H iv, there 

 remains in the skull of the crocodile only the bone 73, which by its position 

 in front of the orbit and its relation to the lacrymal duct, is to be referred 

 like the great anterior suborbital mucous bone in fishes to the dermal skele- 

 ton. In like manner the palpebral or supra-orbital scale-bones are to be ex- 

 cluded from the category of the pieces of the endoskeleton. The small and 

 inconstant ossifications in the capsule of the organ of smell, together with the 

 scarcely ossified sclerotals (17), the small petrosal, 16, and the columelliform 

 stapes, i6', are intercalated portions of sense-capsules and appendages re- 

 ferable to the system of the splanchnoskeleton. 



Thus the endoskeletal system of bones of the head of the crocodile are natu- 

 rally arranged in four segments, each composed of a centrum with a neural 

 and a haemal arch. The haemal arches have been subjected, as in the trunk, 

 to most modification ; that of the occipital vertebra having been displaced; 

 that of the parietal vertebra detached from its segment and arrested in its 

 development ; whilst that of the frontal vertebra is articulated in a very small 

 proportion to the parapophysis of its own segment, but chiefly to that of the 

 parietal segment, with paroccipital connections also ; it is immensely de- 

 veloped, the haemapophysial portion being the chief seat of extension. The 

 haemal arch of the nasal segment is also very large, but shows as much 

 excess of development in breadth as that of the frontal vertebra in length. 

 The diverging appendage is more complex than in fishes : one piece indeed, 

 no. 25, fig. 5, is absent, but three others, 24', 26 and 27, have been superadded. 

 The diverging appendages of the frontal and parietal vertebrae cease to be 

 developed in every class above that of fishes ; but that of the occipital haemal 

 arch, though it no longer shows the luxuriant profusion of rays that distin- 

 guishes it in fishes, begins to assume a more fixed and definite character with 

 more special powers and independent movements of its constituent parts. 

 The first segment (53), doubtfully and obscurely recognizable in any fish, is 

 henceforth a constant and important bone, and is always single : the next 

 segment consists as exclusively of two bones, connate, indeed, in batra- 

 chians : the distal segment presents two jointed rays (digits) in the Amphi- 

 uma didactylum ; three rays in Amph. tridactylum and the proteus and four 

 rays in the Siren lacertina ; it branched into as many as nine rays in the ex- 

 tinct ichthyosaurs ; but they never exceed five in the existing saurians, which 

 number is presented by this appendage in the crocodile (57, fig. 22.) 



Birds — The cranium of the bird offers the extremest instance of a homo- 

 logically compound bone, and its development the clearest evidence of that 

 principle of unity of composition which lies at the bottom of all the modifica- 



