FROM THE LOWER LIAS OF CHARMOUTH. 



451 



be excluded from the cervical series, and there would be only thirty- 

 six cervical vertebras. If it were desirable to make a natural group- 

 ing of the vertebras of this skeleton without reference to those of 

 other species, one would not hesitate to draw the line between cer- 

 vical and dorsal at the end of the thirty-sixth vertebra ; for all 

 down to the thirty-sixth are without transverse processes, but pos- 

 sess hatchet-shaped ribs, while past the thirty-sixth the hatchet- 

 shaped rib disappears and unmistakable transverse processes corre- 

 spondingly arise. As, however, it is convenient to adhere as closely 

 as possible to existing conventions, in order to facilitate the com- 

 parison of species, I have been governed in my determination of the 

 last cervical by the fact that a part of the transverse process borne 

 by it does clearly seem to be contributed by the centrum as far back 

 as the thirty-eighth vertebra ; and thus I have included as cervical two 

 vertebras which would certainly seem more in place in the dorsal 

 series. Professor Seeley's plan of calling those vertebras in which 

 the transverse process is passing from the centrum onto the neural 

 arch "pectoral" has much to recommend it, and might fairly be 

 applied to the vertebras thirty-seven and thirty-eight. Indeed there 

 is just a shade of doubt in my mind whether vertebra thirty-nine 

 should not also be called pectoral ; for its transverse process appears 

 to have a little brown bone like that of the centrum at its base, 

 and the rib it bears is bifurcate near the head and short, while the 

 succeeding vertebras bear ribs with a simple proximal termination 

 only. 



The determination of the position of the last cervical vertebra is not 

 only important as giving us the number of vertebras in the neck, but 

 also because it furnishes us with a necessary datum for the measure- 

 ment of the length of the neck, and hence for ascertaining that im- 

 portant character, the ratio of the length of the head to that of the neck, 

 or, as we may briefly term it, the cervico- cephalic index. Fortunately, 

 in the case of the species under consideration the neck is so long that 

 one or two vertebras more or less can make very little difference to 

 the value of this index, the thirty-seventh and thirty-eighth vertebras 

 measuring together not more than five inches. The thirty-eight 

 cervical vertebras measure 83*25 inches, or 6 feet 11 inches ; and 

 the cervico-cephalic index is 24-1. A Table is here appended, 

 giving the dimensions of the cervical vertebras, so far as they are 

 ascertainable : it will be seen that the constancy in length which 

 Prof. Owen * regards as characteristic of the cervical vertebra of 

 the Enaliosauria, and only exceptionally absent in Pliosaurus, has 

 no real existence, and also that no single vertrebra can well be taken 

 as typical of the remainder. Hence the importance of such a Table 

 as this as a help in the specific identification of separate vertebras 

 will be apparent. It may be worth while to call attention to the 

 abrupt manner in which some of the changes in dimensions occur, as 

 for instance in the length of the centrum in passing from the twenty- 

 second to the twenty-third vertebra ; and again, to the abnormal 



* Eeport Brit. Assoc. 1841. Eeport on British Fossil Kep tiles, p. 63. 



