272 ME. G. E. DIBLEY OK THE TEETH Of PTYCHODVS [May I9H, 



granulated area. In one associated set of six teeth (B.M. 4362) 

 some of the transverse ridges are irregularly subdivided, as in 

 Pt. decurrens var. oweni (PL XVIII, fig. 3). 



Geological range. — This species ranges through the zones of 

 Rhyiichonetta cuvieri, Terebratulina gracilis, and Holaster planus, 

 and may also occur in that of Micraster cor-testitudinarium. A 

 variety from the base of the RliynelioneUa-cuvieri Zone at Bnrham 

 and Jietch worth is characterized by an unusually fine marginal 

 granulation, in which the radiating grooves are scarcely noticeable 

 or are entirely absent (PI. XXII, fig. 5). 



6. Ptichodus rugosus Dixon. (PI. XXII, figs. 6 a & 6 6.) 

 1850. F. Dixon, ' Geology of Sussex ' p. 362 & pi. xxxi, fig. 5. 



Specific characters. — Teeth resembling those of Pt.mammil- 

 laris, but remarkably smooth ; both the transverse grooving and 

 the marginal granulation are feebly marked ; the grooves are fewer 

 than in Pt. mammilla ris. 



Remarks. — The crowns of the typical teeth are rounded, and 

 there is often a characteristic puckering. When they are more 

 flattened and begin to show some distinct surface-markings, they 

 approach the teeth of Pt. mammillaris. In an associated set in the 

 British Museum (P 5335), and in another set in my own collection, 

 the typical teeth occur together with smaller teeth which have an 

 excessively elevated crown and smooth sides. These smaller teeth 

 were erroneously named Pt. altior by Dixon. 1 



Geological range. — Pt. rugosus seems to appear first in the 

 zone of Micraster cor-testitudinarium, and has not been found 

 above that of M. cor-anguinum. I have obtained teeth from the 

 Haling Pit (Purley), Strood, and Gillingham. In the British Museum 

 (Natural History) there are also specimens from Purley Junction 

 Pit (now disused) and Northfleet. Teeth of the same form, from 

 the Niobrara Chalk of the Western States of j^orth America, were 

 named Pt. ivhipplei by J. Marcou. 2 



7. Pttchodus mortoni Agassiz. (PI. XXII, figs. 7 & 8.) 



1839. L. Agassiz, 'Poissons Fossiles ' vol. iii, p. 158 & pi. xxv, figs. 1-3. 



Specific characters. — Central part of crown of teeth raised 

 to a point, from which grooves and ridges radiate irregularly; 

 marginal area wide, and finely granulated. 



Remarks. — Although common in the Cretaceous of North 

 America, 3 whence the type-specimen was obtained, this form of 

 tooth is extremely rare in the English Chalk. 



Dixon 4 figures a specimen from Shoreham, and Dr. A. S. Wood- 

 ward 5 another from Winchester; apparently the crown alone of 



1 ' Geology of Sussex ' 1850, p. 362 & pi. xxx, fig. 10. 



2 ' Geology of North America ' Zurich 1858, p. 33. 



3 S. W. Williston, Kansas Univ. Geol. Surv. vol. vi (Palaiont.) pt. ii (1900) 

 pp. 234-40. 



4 ' Geology of Sussex ' 1S50, p. 364 & pi xxxi, figs. 6-7. 



5 Proc. Geol. Assoc, vol. xiii (1893-95) p. 191 & pi. v, fig. 4. 



