I. THE GENUS DIMORPHOCERAS The genus Dimorphoceras was established by Hyatt ( 6 , p. 331) on the evidence afforded by the illustrations and descriptions of Goniatites gilbertsoni Phillips ( 12 , p. 236, pl. 20, figs. 27–31) and G. looneyi Phillips ( 12 , p. 236, pl. 20, figs. 32–35). In 1897 Foord and Crick ( 5 , p. 219) selected the first-named species as the genotype, and of this species they refigured one of Phillips’ type-specimens ( 5 , p. 221, text-fig. 105a) along with an end-on view and a suture line drawn from other specimens; unfortunately the locality has not been recorded but it is supposed to be in the neighbourhood of Bolland. Dr. L. F. Spath has kindly compared specimens from the Beyrichoceras beds at Dinckley with type material in the British Museum and says these are similar, so that it can be taken as probable that the horizon of D. gilbertsoni (Phillips) is high in Zone B. Previous diagnoses of genus: Hyatt ( 6 , p. 331) defined the genus as follows:—“ Species with involute compressed whorls and sutures quite distinct from those of Nomismoceras on account of their peculiar siphonal saddles, narrow first pair of saddles and divided lobes, but resembling them closely in their magnosellarian saddles and general aspect. They are in fact only more complicated and modified examples of the same style of sutures, the lobes having ammonitic marginal saddles in place of entire outlines. They have a narrow, prominent, siphonal saddle, and minute funnel lobe, the arms of the ventral lobe …