AND AFFINITIES OF PTYCHODTJS. 



125 



adapted to the next is again very evident, and there is the same 

 slight accommodative variability in shape. The superficial ornamen- 

 tation is also equally constant ; but there is a greater tendency to 

 obliquity in all the teeth, this becoming observable even in series 

 no. 2, and especially marked in the rows beyond. 



IY. A small example (fig. 3) of the same species from the Lower 

 Chalk of Hailing, Kent, affords the first definite indication of the 

 second plan of arrangement already referred to. Only nine teeth 

 are shown in their natural relative positions, forming parts of three 

 parallel rows ; but in the absence of further evideuce, the remarkable 

 want of symmetry is a most perplexing characteristic. The middle 

 series, represented by four of its components, is considerably the 

 largest ; but in the two adjacent rows the teeth of the one side are 

 notably smaller than those of the other, as shown by the following 

 average dimensions : — 



Series. Transverse Measure. Anteroposterior Measure. 



0 0-006 0-005 



1 0-014 0-010 



2 0-010 0-007 



It is also noteworthy that in the smallest teeth the peculiarities 

 of the grin ding-surface are somewhat abnormal, the peripheral 

 granulated area encroaching to such an extent upon the transversely 

 furrowed portion that the latter becomes quite inconspicuous. 



Y. But a complete explanation of this specimen is afforded by 

 Mr. Willett's magnificent fossil from the Chalk of Brighton (PI. X. 

 figs. 4-10). Like most of the other comparatively undisturbed re- 

 mains, it is referable to the common P. decurrens, and no less than 

 thirty-four teeth are firmly cemented together in their natural rela- 

 tions by slightly crystalline pyrites ; while seventy others were 

 originally associated with this mass, though now detached in small 

 groups and isolated specimens. 



Of the jaw already described, only seven teeth are cemented in the 

 main mass, and these belong to the median and the first left lateral 

 row ; but of the opposing jaw, hitherto entirely unknown, there are 

 satisfactory remains of six longitudinal series, and certain detached 

 fragments afford evidence of still more. Here, again, there is com- 

 plete symmetry around a median azygous row ; and in describing the 

 arrangement it will obviously be necessary to use the terms right 

 and left in the opposite sense to that already employed in reference 

 to fig. 1. 



The median series is scarcely visible on the grinding- surface, only 

 a portion of one tooth projecting through the hard pyrites ; but evi- 

 dence of its presence is afforded by the distinct abrasion of the sum- 

 mits of the opposing teeth, and one of its components is exposed on 

 the back of the fossil, while two others are shown in a detached 

 fragment. These teeth (fig. 5) are comparatively small, elongated 

 antero-posteriorly, and, like those of the most diminutive row in 

 specimen No. IY., are somewhat abnormal from the circumstance 

 that the granulated ornament considerably encroaches upon the 



