Vol. 69.] THE 'KELLOWAY ROCK ' OF SCARBOROUGH. 165 



advisable to fix one as the type, and for this the example depicted 

 in his figs. 5 & 6 is selected. I understand that this occurs high 

 in the Divesian ; but it is doubtful whether we have it in England. 

 The Russian variety (figs. 7 & 8) has nothing to do with it, and 

 probably occurs low in the Callovian. See Gadoceras grewingM 

 (p. 162). 



Quknstedtoceras placenta. This very involute species is not a 

 true Qmnstedtoceras, but still less is it a Cadoceras of the grewingTci 

 series. It would seem, to be peculiar to Yorkshire. 



Qlenstedtoceras TURGiDUM, This is a senile .form, like the 

 example which A. d'Orbigny has ascribed to Ammonites lamberti 

 (' Terrains jurassiques : Cephalopodes ' pi. clxxviii only), but with 

 a more trigonal whorl, and the venter is more acute in the costate 

 stage. 



Quenstedtoceras vERTT/MNtrs. This is a species with stout ribs, 

 which look like pieces of cord wound round the whorl. 1 have not 

 seen it from any other locality. Dr. Pompeckj rightly associates it 

 with Q. marice d'Orbigny sp., but it is not 'a modification of a 

 Qaenstedtoceras marice d'Orb. sp. with a wider umbilicus' : ' rather 

 is Q. marice the involute inflated development of Q. vertumnus? 



IV. On Development and Homceomorpht. 



There is an interesting repetition in development, leading to 

 homceomorphy, in many of the Middle Jurassic ammonites. They 

 pass repeatedly and independently from evolute compressed to 

 involute inflated, in some cases to sphterocones, a phenomenon 

 which may be observed in the Callovian genera Cadoceras, Phlyeti- 

 ceras, in the Divesian Qaenstedtoceras, and in the Argoviau Cardio- 

 ceras. (A similar line is followed by the Kimmeridgian Amceboceras, 

 but is not carried so far.) The same phenomenon is also repeated 

 again and again in the different stocks of these various genera quite 

 independently. This phenomenon has caused much confusion in 

 the identification of the various species, and has led to much 

 ' lumping,' because it was not understood. And the ' lumping,' or 

 the failure to recognize what were the critical points of distinction, 

 underlying the likeness, has caused stratigraphical lists to be in- 

 valid, and has also given a wholly false range to some much-quoted 

 species, thereby impairing stratigraphical correlation. And that 

 the likeness, the homceomorphy, should have caused the ' lumping 

 is the best testimony to its completeness. The likeness is often the 

 greater when, as is so frequently the case, the loss, or nearly so, 

 of the principal distinctive characters (ornament, keel, etc.) has 

 accompanied the inflation. 



Other genera not dissimilar in time and in appearance, Macro- 

 ceplialites,Erymnoceras,Pacliyceras, develop in a different direction — 



1 J. F. Pompeckj, 'Jurassic Fauna of Cape Flora': Norw. Worth Polar 

 Exped. vol. i (1900) No. 2, p. 97. 



- See § IV, 'On Development & Homceomorphy.' 



