162 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. ITI. 



implantation of the teeth, viz. : 1, in distinct sockets, as in 

 Crocodiles ; 2, in a continuous groove or furrow, as in the 

 Ichthyosauri ; 3, attached laterally by the shank to the 

 alveolar parapet, as in the Iguanas (pleurodonts) 4, anchy- 

 losed to the base of a shallow socket, (thecodonts) ; and 5, 

 attached to an osseous support without sockets or an alveolar 

 parapet (acrodonts), as in the Mosasaurus. 



The compound structure of the lower jaw of reptiles is 

 also peculiar ; there are six distinct bones on each side, and 

 these elements undergo various modifications of form and 

 arrangement in the respective families. 



VERTEBRA OF REPTILES. The vertebrae, as the elements 

 which form the spine are termed, are the most numerous and 

 important parts of the skeletons of the extinct reptiles, that 

 come under the observation of the palaeontologist, and which 

 present in the gigantic terrestrial saurians, extraordinary de- 

 viations from the homologous bones in the existing species. 



Unfortunately, connected series of vertebrae are but rarely 

 met with in the fluviatile deposits in which the exuviae of the 

 most remarkable land lizards occur ; and it is, therefore, often- 

 times extremely difficult to determine whether cervical, dorsal, 

 and caudal vertebrae, discovered apart from each other in the 

 rocks, belong to a spinal column typical of one species or 

 genus, or to several genera. Hence specific and generic 

 distinctions founded on isolated bones, can only be admitted 

 as provisional ; for in more than one instance different parts 

 of the same vertebral column have been biade the basis of 

 three distinct genera ; and, on the other hand, a supposed 

 generic vertebral character not unfrequently proves to be 

 distributed through an entire family of saurians. 



The fragmentary and isolated condition of the vertebras 

 imbedded in the Wealden strata of the South-East of England, 

 is a sufficient excuse for error in the interpretation of a piece 

 of bone, or in the reference of an entire bone to a particular 

 species or genus. My own mistakes in this respect I have 

 always unreservedly pointed out; and it is much to be 

 regretted that other labourers in the same department of 

 natural history, of far higher pretensions, do not imitate the 

 noble example of the illustrious Cuvier, and admit and 

 correct the errors into which they have fallen by the ascrip- 

 tion of specific and generic distinctions without sufficient 



