and apparently with good reasons for so doing, have proved to be only 

 different stages of growth of the same fshell. 



The old genus Ammonites of Bruguiere, with its eight or nine hundred 

 of so called species, is a heterogeneous assemblage, which requires division 

 into several genera and subgenera. 8toliczka saj^s, very truly, that the 

 animals of Turritella and Cerithium are not in any way more different 

 than must have been those of Ammonites discus and A. Rotomayensis. The 

 Avhole of the group has been revised anew, on the j)rinciple just indicated, 

 b}^ Dr. Waagen and others in Cxcrman}', and by Prof. A. Hyatt, in 

 America. The new generic or subgeneric names proposed by these 

 authors will be adopted in this memoir, at least in those cases in which 

 there is a i-easonable probability of their being correctly applied. 



Considerable differences of opinion have existed, and probably always 

 will exist, with regard to what constitute specific differences in these shells. 

 Those whose experience has been gained by a study of many specimens 

 in the held, naturally attach less importance to minute differences in form, 

 surface markings and the like, than is accorded to them by others whose 

 opportunities for extended compiu-isons have been few. In this connec- 

 tion the late Prof Phillips justly remarks* : — "The zeal of collectors, by 

 procuring them (Ammonites) of all ages and under different circum- 

 stances, has given occasion to coin too great a number of specific names. 

 Yet for the most part, the diversity of names for a given set of forms 

 indicates something really different in the history of the species, and most 

 of the designations ma}' be retained as marking varieties worth discrimi- 

 nation. In making, some years since, a strict comparison of the 

 ammonites of the Yorkshire lias with others from the south of England 

 esteemed to be of the same species, I found often some small differences, 

 especially in the sutures, which might be best understood as local pecu- 

 liarities of race. Ammonites to be really known as species, must be 

 studied with many examples of every age, including the very young and 

 the very old; the change of form in the course of life being often very 

 great and remarkable." 



In the present instance it has been impossible to comply with the con- 

 ditions stated in the last sentence of the above quotation. Several of the 

 species in this genus, for which new names will be pro2)Osed in these 

 pages, are founded on a single imperfect specimen, and in no case has a 

 large series been obtained. The same, indeed, may be said of all the shells 

 in the collection. The septation, too, which, when properly studied, is 



* "Geology of Oxford and the Valk'j of the Thaiiieis." Oxford : ls71. Page 131, 



