468 PEOF. W. J. SOLLAS OK A NEW SPECIES OF PLESIOSATTBTJS 



Classification. 



The pectoral arch in its essential characters is truly Plesiosaurian, 

 though it differs from most Plesiosaurs in the fact, if it be a fact, 

 that the coracoids do not extend in front so as to overlap the 

 " lapels " of the furculum. 



To make plain its relations to other members of the genus Plesio- 

 saurus, I have constructed the following Table, in the last four 

 columns of which the distribution of the vertebrae amongst the 

 various regions of the spine is given for each species. 



In comparing the length of the head with that of the neck (or 

 rather of the cervical series of vertebrae) I have uniformly made use 

 of the proportion : — 



length of head I 

 length of neck - " 100* 



The value found for I may be conveniently called the cervico- 

 cephalic index. This index is given, for each species in which it has 

 been determined, in the first column of the Table. 



In the second column I have similarly compared the length of 

 the head and the dorsal series of vertebrae (excluding sacral vertebrae). 

 The indices of this column are dorso-cephalic. 



The total length has not been made use of in comparison, since it 

 is seldom possible certainly to obtain it, and variations in the length 

 of the caudal region are of secondary value ; but in the third column 

 values are given for the length of the head compared with the cer- 

 vico-dorsal series (exclusive of sacral vertebrae). To the cervico- 

 dorsal cephalic index great value may be attached, since it can 

 frequently be obtained and is independent of any error in the method 

 or the practice of determining the position of the last cervical ver- 

 tebra. It is true it may be affected by a mistaken determination of 

 the first sacral vertebra ; the sacral vertebrae, however, are more 

 easily determined than the last cervical ; and an error with regard 

 to them will not cause a deficiency or excess of more than one or 

 two vertebrae to the cervico-dorsal series, while I am convinced 

 much larger errors have been made with reference to the number 

 of vertebrae in the cervical series ; and, finally, in comparatively so 

 great a length as that of the cervico-dorsal series one or two vertebrae 

 more or less will have but a trifling effect on the value of the index 

 obtained from them. Another index of some interest is the cervico- 

 dorsal, obtained by referring the length of the neck to that of the 

 trunk taken as 100. 



The values now given for the various indices will certainly in 

 many cases need revision, since the measurements on which they 

 are founded are often extremely unsatisfactory. Sometimes they are 

 unreliable or erroneous, sometimes vague (as when the length of the 

 " trunk " is given without stating whether the sacral vertebrae are 

 included or not, or when the length of one part is given in fractions 



