Vol. 67.] AND THEIR DISTRIBUTION IN THE ENGLISH CHALK. 27l 



In 1898 I discovered in the Terebratulina-gracilis Chalk afc 

 "VVhyteleafe a tooth which I failed to recognize ; subsequent dis- 

 coveries of an associated set of thirty-six teeth in the same zone at 

 Cuxton, in which the features exhibited by the Whyteleafe tooth 

 were persistent throughout the set (PI. XX, fig. 1), and of another 

 associated set of fourteen teeth from the same locality, have con- 

 firmed my reasons, with which Dr. A. S. Woodward agrees, for 

 treating this as a new species. 



Specific characters. — Shape: lateral teeth more oblong than 

 in the previously described species ; central teeth of large inner 

 paired row of (presumably) upper dentition nearly square. Grooving 

 not nearly so coarse as in Pt. latissimus, but more so than in Pt. 

 polygyrus, with a tendency to occupy the whole surface of the 

 lateral teeth and a curving of the ends towards the anterior portion 

 of the tooth, as in Pt. latissimus. The granulated area is less than 

 in Pt. latissimus and Pt. poly gyrus var. marginalia, notably in the 

 anterior portion of the lateral teeth, while in the big central teeth 

 the granules are very coarse where they adjoin the last groove ; 

 this area is somewhat increased in these large teeth. 



The evidence from which these features were obtained is (1) the 

 tooth already mentioned ; (2) the associated set (B.M. P 10260) ; 

 (3) an associated set of eleven large teeth — in all probability from 

 the first inner paired rows of the upper dentition, — and three 

 smaller teeth belonging to the same set from the Terebratulina- 

 gracilis Zone at Cuxton and kindly lent by Mr. J. G. Wilson for 

 comparison with the other examples : one of these large teeth is 

 shown of the natural size in PI. XX, fig. 2 ; (4) a tooth recently 

 obtained from the same locality ; (5) an associated set from Lewes 

 (B.M. 4355) ; and (6) an associated set, Dixon Collection (B.M. 

 28339). 



Geological range. — This species has been so far recorded 

 only from the zone of Terebratulina gracilis, and probably from 

 that of Holaster planus. 



5. Ptychodus mammillaris Agassiz. (PL XXII, figs. 3-5.) 



1839. L. Agassiz, ' Poissons Fossiles ' vol. iii, p. 151 & pi. xxv b, figs. 12-20 



(non 11). 

 1850. Ptychodus latissimus F. Dixou, ' Geology of Sussex ' pi. xxxi, fig. 3 



{err ore). 



Specific characters. — Central part of crown of tooth sharply 

 raised. Transverse ridges numerous, extending directly across the 

 raised part and continued down its sides, sometimes tending to be- 

 come gyrate at the ends. Granulated marginal part of the tooth 

 very wide, fine, usually crossed by a few shallow radiating 

 channels ; granules tending towards a concentric arrangement. 

 Lateral teeth very unsymmetrical (PL XXII, fig. 4). 



Remarks. — The four type -specimens in the Mantell Collec- 

 tion are isolated teeth with remarkably flattened crowns. Others 

 are more rounded at the summit of the raised portion. Many 

 specimens exhibit some approach to the teeth of Pt. polygyrus, but 

 they can always be distinguished by the character of the marginal 



Q. J. G. S. No. 266. t 



