1. Ptychodus Mammillaris, Agassiz. (Figs. 3— 14.) 
An associated series of eighty teeth from the Chalk near 
Rochester, Kent, is referable to the well-known P. mammillaris ; 
and all the principal series of each jaw seem to he represented. 
None of the teeth are in natural order, and only the median 
rows can thus he determined with precision. Of the small teeth 
of the upper median series, there are three examples, and one is 
shown in fig. 4. This tooth has not previously been described 
and figured, and on comparison with the corresponding teeth of 
other species already known, it will he observed that there are 
well-marked distinctive features. As shown in end view (fig. 
4a) , the root exhibits the usual great relative depth ; the crown 
is less elongated antero-posteriorly than in P. decurrens* and 
P. latissimus,-\ though more so than in P. ])olygyrusX and 
P. rugosus ; § and the coronal surface is remarkably smooth, 
the peripheral area being faintly granulated, and the median 
raised area being very small, crossed by three short ill-defined 
ridges. The large median lower tooth is shown in fig. 3 ; and 
both this and the unsymmetrical principal lateral teeth (figs. 5— 
11) exhibit, with remarkable uniformity, all the characteristics 
of the species,—the much elevated median area, with numerous 
regular transverse ridges, sharply defined from the large 
peripheral area, and the latter marked by delicate rugae, mostly 
elongated and concentric with the borders of the crown, and 
occasionally crossed by shallow radiating grooves. The tooth 
represented in fig. 5 is almost certainly referable to the first 
lateral series of the upper jaw, as indicated by its relatively 
large size ; and the original of fig. 6 may have occupied a 
similar position in the lower jaw. The outermost lateral teeth 
(figs. 12—14) are scarcely distinguishable from those of other 
species ; but even in these teeth, the characteristic elevation of 
the central area is sometimes prominent (fig. 14a). 
The finest group of teeth of Ptychodus mammillaris hitherto 
discovered, is now preserved in the Woodwardian Museum, 
W. Buckland, “ Greology and Mineralogy, Ed. 2 (1837), PL xxvii.y. 
^ Smitli Woodward, Quart. Journal Geol. Soc., vol. xliii., PI. x„ fig. 5. 
t Ihid ., PI. X., fig. 12. 
J Ibid ..^ PI. X., fig. 11. 
§ Catalogue FlssiI Fishes British Museum, pt. i., PI. v., fig. 3. 
