6 FOSSIL REPTILIa OF THE LONDON CLAY. 



is figured in T. XI, will show that it consists mainly of a series of segments, more or 

 less alike. From the back of the head to the end of the tail, the chief part of each 

 segment consists of a cylindrical portion or ' body,' differing only in its proportions, 

 and diminishing as it recedes from the trunk. Every segment sends a plate of bone 

 upwards from its upper or dorsal surface, which plate or ' spine' is supported by an 

 arch of bone, except in the diminishing segments at the end of the tail. 



Other plates of bone, of more variable forms and dimensions, project from each side 

 of the segments of the trunk and basal part of the tail. In a less proportion, but still 

 in a great number of the segments, an arch of bone is formed below, or on the ventral 

 side of the cylindrical body ; but this lower arch is more variable in its proportions 

 and mode of composition than the upper arch : it is open or incomplete in the neck. 

 Under all these variations, however, there is plainly manifested a fundamental unity 

 of plan in the composition of the different segments, which have accordingly received 

 the common appellation of l vertebra.' 



For the convenience of description, the vertebras are divided, though somewhat 

 arbitrarily, into groups bearing special or specific names. Those next the head, with 

 the inferior arch incomplete below, are called ' cervical vertebras;' they are usually muc- 

 in number : those that follow with the inferior arch closed below, or which have the 

 laterally projecting parts slender and freely moveable, are called ' dorsal vertebras;' 

 the other vertebras of the trunk that have no lateral moveable appendages, are called 

 ' lumbar vertebras ;' the last vertebras of the trunk, always two in number in the 

 Crocodilia, the inferior arches of which coalesce to support and be supported by the 

 hind limbs, are the ' sacral vertebras ;' the segments of the tail are the ' coccygeal,' 

 or ' caudal vertebras/ whether they possess or not an inferior arch, or whatever other 

 modifications they may offer. 



These names, ' cervical,' ' dorsal,' ' lumbar,' ' sacral,' ' coccygeal,' were ori- 

 ginally applied to corresponding segments or vertebras in the human skeleton, from the 

 study of which the nomenclature of osteology takes its date : it may well be supposed, 

 therefore, that a classification and designation of vertebras based upon knowledge 

 limited to their characters in a single example of the vertebrated series, and that 

 example one in which the common type has been most departed from, to adapt it to 

 the peculiar attitude and powers of the human species, would fall far short of what 

 is required to express the general ideas derived from a comparison of all the leading 

 modifications of the vertebrate skeleton ; and accordingly the anatomist who passes 

 from a previous acquaintance with human osteology only, to the study of those of the 

 lower Vertebrata, finds that he has to rectify, in the first place, the erroneous notions 

 which anthropotomy has taught him of the nature of the primary segment of his own 

 and other vertebrated skeletons, and to acquire true ideas, with the concomitant 

 nomenclature, of the essential constituents or anatomical elements of such segment. 

 In human anatomy, for example, the costal elements are only recognised when they 



