PTYCHODUS. 233 



Type. — Detached teeth ; some in British Museum. 



Specific Characters. — The largest known species, of which the lower median 

 teeth sometimes measure 8 cm. in transverse diameter. Median portion of tooth 

 not much raised, but flattened and crossed by coarse transverse ridges, which 

 often curve round at the lateral ends and sometimes form loops. Marginal area 

 of variable width, more or less coarsely granulated, often showing some concentric 

 arrangement but never crossed by radiating grooves. Transverse ridges of lower 

 median teeth from 11 to 14 in number. 



Description of Specimens. — The teeth commonly referred to this species are 

 so varied in shape and markings that it may be doubted Avhether they are 

 rightly grouped together. Agassiz, indeed, originally recognised, besides the 

 typical form, three varieties which he named concentricus, sulcatus and marginalis 

 respectively. Of these, the single tooth figured as var. concentricus is most 

 probably an abnormal example of P. rnammillaris, while the two teeth named var. 

 sulcatus seem to be referable to P. decurrens. Between the typical P. polygyrus 

 and the variety marginalis, however, there appear to be all gradations; and as 

 the two forms have been proved by Mr. Dibley to occur in two successive 

 horizons, it is probable that the one is a mutation of the other. 



The typical form of P. polygyrus is found in the higher horizon, and is best 

 known by an associated set of teeth discovered by Mr. W. Murton Holmes in the 

 zone of Micraster coranguinum in Surrey. These teeth were scattered in a block 

 of chalk, not arranged in natural order ; but the selection shown in PI. XLIX 

 probably illustrates the principal rows of both jaws correctly identified. The 

 small upper median teeth (fig. 1, o') are considerably longer than broad, with 

 truncated anterior and posterior ends, and a remarkably deep root. Their coronal 

 face is occupied chiefly by the finely granulated marginal area, the middle trans- 

 versely-ridged portion, though variable, being always A r ery small and not much 

 raised. The transverse ridges are from four to six in number, and variously 

 irregular or interrupted. The large upper teeth of the inner paired row (V) 

 agree closely in size, shape, and markings with the original typical teeth figured 

 by Agassiz (loc. cit., pi. xxv, figs. 10, 11, pi. xxv b, fig. 23). They bear nine or ten 

 transverse ridges, which are more or less irregular, but always tend to converge in 

 gyrations at the lateral borders. Their narrow granulated area is widest at the 

 anterior and external margins. The teeth which seem to belong to the upper 

 second paired row (n') are much smaller, and are crossed by seven to nine 

 transverse ridges, which leave space for scarcely any granulation except at the 

 anterior margin. The internal margin is sharply truncated, while the outer 

 margin slopes gradually downwards to the plane of the next outer row. Some 

 of these teeth exhibit a tendency to oblique distortion, which is still more marked 

 in those of the next row (in'). Here the teeth are much smaller, with only six 

 or seven transverse ridges, but their internal margin is again more sharply 



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