IIYPERLIOCERAS WALKERI. 93 



to delineate the adult of each variety since the differences can be indicated verbally 

 with the aid of that figure. The forms may be conveniently classed as follows : 



a. (Plate XVI, figs. 1 — 4.) Median, typical form. 



/3. (Plate XVI, figs. 5 — 9.) Very flat form, with wider umbilicus. 



y. (Plate XVI, figs. 10, 11.) Thicker than a, with smaller umbilicus. 



In considering the comparative width of the umbilicus the amount of body- 

 chamber must be taken into consideration. For instance, fig. 5 has no part of 

 the body-chamber present ; but fig. 1 is noticeable for the great increase in the 

 width of the umbilicus after the body-chamber commences, and even a little 

 before. The cross marks the position of the last suture, and, when we consider 

 that half a whorl from that point may be reckoned for body-chamber, and that 

 the increase in width continued from that period at the same ratio, we see the 

 great difference which the presence of the body-chamber makes in the size of the 

 umbilicus, since the inclusion at the mouth would be barely half a whorl. 



This sudden umbilical expansion 1 and the strong carina distinguish this species 

 from any of the Liocerata, such as Lioceras decipiens. The young forms are not 

 so easily separated from Lioc. intermedium, but are more compressed on the outer 

 lateral area, and possess a more distinct carina on a flatter ventral area. The last 

 sentence of the paragraph explaining the differences between Lioc. intermedium 

 and this species (page 34, lines 23 and 24 from top) became somewhat involved in 

 passing through the press. What I intended to say was, that in making com- 

 parisons between the two species as regards the ventral area care should be taken 

 to choose specimens from the one species which possessed an amount of body- 

 chamber similar to that which the specimens of the other species had, or else that 

 the specimens of both should be without body-chamber, since it is not right to 

 compare the ventral area and carina on the body-chamber of Hyperlioc. Walker i 

 with the ventral area and carina on the air-chambers of Lioc. intermedium, for the 

 reasons which I had then just given. 



The umbilicus in this species is wider than that of the other species of the 

 genus, and is wider than we should expect to find in course of development from 

 its ancestor. 2 This may be a case of reversion, since the development of the full 

 number of auxiliary lobes would appear to indicate descent from a highly involute 

 Hyperlioceras (in the Murchisonai-zone), which had improved, by development of 

 more auxiliaries, on the suture-line of Pseudolioceras. Without the intervention 

 of a highly involute Hyperlioceras there would have been no reason for the 

 development of more auxiliary lobes, because Hyperlioc. Walkcri, with its narrower 

 whorls, 3 would not have required them. 



The horizon of this species is the Concavum-beds, at Bradford Abbas, Dorset ; 



1 None of the Liocerata which I have seen show such an expansion as this would be. Lioc. 

 decipiens shows nothing like as much. 2 See p. 139. 



3 That is, narrower when compared with what usually obtaius in Pseudolioceras and Hyperlioceras. 



