Vol. 6g.~] THE ' KELLOWAY ROCK ' OE SCARBOROUGH. 163 



plate (' Min. Conch.' vol. i, 1812, pi. liv), leaving the more globular 

 form as a matter of convenience under the name O. modiolare 

 Lhywd-d'Orbigny sp., of which A. d'Orbigny's figure l would be 

 the type. 



From Gristhorpe Bay I have a fragment of C. sublceve as above 

 defined. It is from a matrix of loose sand, suggesting Leckenby's 

 bed B, but he says that this is unfossiliferous. 



C. sublceve ( + modiolare) would appear to be easily identifiable; 

 but in one case it has been very much misunderstood (see below, 

 Pacliyceras robustum). 



Pachxceras regostjm. Bean's Ammonites rugosus is a species of 

 the rare genus Pacliyceras, differing sufficiently from A. lalandeanus 

 d'Orbigny. I cannot recall any other record of an English species 

 of the genus. 



Pachyceras robustum, nom. nov. This is founded on Stepliano- 

 ceras sublaive, E. E.-Deslongchamps, ' Eoss. Oxford. Coll. Jarry ; 

 jNTot.es Paleontologiques,' vol. ii (1889) pi. i, figs. 2-4, taking- 

 fig. 4 as the type. Prof. Pompeckj 2 has noticed that the species 

 is not identical with Sowerby's Am. sublcevis, but he has failed to 

 see that it is not a Cadoceras, for he has definitely assigned it to 

 that genus. However, the straight massive ribs and the nodose 

 ornament strong on the umbilical border show that it is not a 

 Cadoceras : it belongs to the genus Pacliyceras, and like this genus 

 but unlike Cadoceras it becomes less inflated with age. 



P. robustam is near to P. rugosum, but has more pronounced 

 ornament and strong umbilical nodosities, of which P. rugosum 

 shows only traces. P. rugosum is also thinner. The position 

 of P. robustum, lamberti beds of Villers-sur-ller (= lamberti- 

 gregarium zones, perhaps, of this paper), is correct for a Pacliy- 

 ceras, but is incorrect for a Cadoceras — a genus which belongs to 

 the Lower Calloviau. 



P. robustum shows that the genus Pachyceras is an offshoot of 

 the genus Enjmnoceras : the study of P. rugosum had already led 

 me to form the same conclusion. P. robustum, P. rugosum, 

 P. lalandeanum form in this order a good catageuetic series, in 

 which ornament and thickness are decreasing. 



The identification of P. robustum with Ammonites sublams and as 

 a Cadoceras is good testimony to the deceptiveness of homoeomorphy 

 in certain cases, and the likeness thus testified to is an example of 

 a rather unusual form of homceomorphy (see § IV, ' On Development 

 & Homceomorphy ' p. 165). 



Peltoceras MURRAYANEir. There must be some error in Simpson's 

 description given by Leckenby (op. cit. p. 10). To describe a 

 species of this genus as 12 inches in diameter but only three- 

 quarters of an inch thick, must be incorrect ; yet the description 



1 'Terrains jurassiques: Ceplialopodes ' 1842-49, pi. clxx. 

 - 'Jurassic Fauna of Cape Flora' JNorw. North Polar Exped. vol. i (1900) 

 No. 2, p. 79. 



