240 FOSSIL FISHES OF THE ENGLISH CHALK. 



which represents several teeth of both jaws cemented together in their natural 

 position by iron pyrites. The rows of teeth of the upper jaw (marked i' to m') 

 are exposed from their oral face, while some middle teeth of the lower jaw 

 (marked o, 1) are seen still in position opposed to them, and a few detached teeth 

 are drawn separately. The upper median row of small teeth is nearly covered by 

 pyrites, but one partially free example (o') is shown in upper and anterior view. 

 This tooth is tumid, longer than broad, and narrowed in front, its bluntly rounded 

 anterior end fitting into the slightly hollowed posterior end of its predecessor. 

 Its finely granulated marginal area is wider than in the other teeth, its few regular 

 transverse ridges being confined to a small central portion. The teeth of the 

 upper inner paired row (V) are a little broader than long, with the crown raised 

 into a gently arched dome. They are crossed by about ten regular transverse 

 ridges, passing gradually at their ends into the narrow marginal area, in which the 

 fine granulations distinctly tend to an arrangement in lines at right-angles to the 

 margin. The teeth of the upper second paired row (n') are scarcely more than 

 half as broad as those of the first, while those of the third and fourth rows (in', iv') 

 are still much smaller. The latter teeth are a little obliquely distorted, sometimes 

 with an extension or expansion of the postero-internal angle. There were probably 

 one or two more external rows, but these are not preserved. The lower dentition 

 resembles the upper dentition in being marked with regular transverse ridges 

 passing into radiating rows of marginal granulations ; but the middle portion of 

 the crown in all the teeth is more sharply raised than in those of the upper jaw. 

 This elevation is specially marked in the symmetrical teeth of the lower median 

 row (o), which are the largest teeth in the mouth and are crossed by about twelve 

 transverse ridges. In the teeth of the lower inner paired row (i) the elevation is 

 nearer to the outer than to the inner margin, and the whole crown is very 

 unsymmetrical, often with the postero-internal angle irregularly produced. All 

 these teeth are much broader than long, but those of the lower second paired row 

 are more nearly equilateral, and their crown is both less elevated and less uniform 

 in shape. The teeth of the more external lower rows are not identifiable. The 

 average transverse measurements (in millimetres) of the teeth of the several rows 

 are as follows: o', 9; i', 25; n', 16; in', 14; iv', 10; o, 32 to 36; I, 26; if, 16. 



The small specimen already described as showing the cartilage of the jaws 

 (Text-fig. 70, p. 226) also displays teeth of the typical form of P. decurrens ; but 

 the relative proportions of the teeth of the several rows are somewhat different 

 from those above mentioned, as indicated by the following average trans- 

 verse measurements (in millimetres) : o', 4; i', 6*5 ; n', 5; in', 4; iv' 3'5; v', 3; 

 o, 8; i, 6*5; n, 5; nr, 4; iv, 8; v, 2. This fossil may represent a young indivi- 

 dual, but it is noteworthy that in the lowest zones of the Chalk some of the teeth 

 are exceptionally small, smooth, and feebly marked. Such specimens (PI. LI, 

 figs. 13, 14) have been described under the name of P. levis (A. S. Woodward, 



