124 



MK. A. S. WOODWAKD ON THE DENTITION 



represented by five teeth, but of its homologue to the right only 

 four remain. These teeth also vary but little in size, and their 

 average measurements are 0-013 in a transverse direction, and 0*01 

 autero-posteriorly. They are not adjusted to the edges of the inter- 

 posed large teeth with any marked regularity, having no definite 

 relation to the interspaces ; but each example appears to be slightly 

 adapted in shape to fit whatever position it occupies. A second 

 lateral row (2), similarly adapted to the outer border of the first, is 

 indicated by six of its components to the left aud by two to the 

 right ; these are still smaller in size than the others, measuring 

 only 0*01 by 0-009 on the left, though the two corresponding teeth 

 situated more posteriorly on the right are a millimetre longer, and 

 thus square. A still smaller and somewhat oblique series (3), forming 

 a third lateral row, is represented to the right by a single tootb, the 

 dimensions of which are only 0*008 by 0-008. 



This specimen affords an excellent opportunity of observing the 

 character of the surface-ornamentation of the teeth in different parts 

 of the mouth, and entirely confirms the conclusions of Agassiz based 

 upon scattered groups. There is no variation of importance, and 

 such as would lead to the establishment of more than one species if 

 the teeth were found detached ; and, except in size, the lateral teeth 

 only differ from the median in being slightly flatter, and, in a few 

 cases, somewhat oblique. 



III. Another fossil in the British Museum that may perhaps be 

 referred to the same jaw as the preceding, though quite as probably 

 belonging to the opposing dentition, is also of considerable import- 

 ance, and is shown of the natural size in PI. X. fig. 2. This is an un- 

 doubted example of P. decurrens, obtained from the Chalk of Dorking, 

 and forms part of the late Dr. Bowerbank's collection (B. M., no. 

 39134). Twenty-one teeth are preserved in their natural relations, 

 and these are the representatives of six parallel rows. The largest 

 series, to the extreme right, is only indicated by a single one of its 

 components ; but of the second row there are three ; of the third, 

 six ; of the fourth, six and the impression of a seventh ; of the fifth, 

 three ; of the sixth, two : and on the back of the fossil there are 

 several scattered examples of a seventh row (fig. 2, 7). In each of 

 these rows there is exceedingly small variation in size, and the 

 maximum transverse and longitudinal measurements are as follow : — 



Series. Transverse Measure. Anteroposterior Measure. 



1 0-023 0-017 



2 0-015 0-012 



3 0-011 0-009 



4 0-009 0-007. 



5 0-008 0-006 



6 0-006 0-005 



7 0-005 0-004 



The teeth are slightly more displaced than in specimen No. II. ; 

 but the irregular manner in which the components of one series are 



