Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 157 



the mouth there is a set of strong, plain, rounded ribs without any 

 tubercles, which pass over the back and along the sides till they 

 nearly meet the ribs which start from the umbilical tubercles. 



It is most nearly allied to A. Deverianus, but this species, as 

 figured by d'Orbigny and Sharpe, differs in the following particulars : 

 it is much less involute and more inflated than A. pentagonus, it 

 has a much wider umbilicus, there is an extra row of tubercles 

 along the middle of each side of the shell, and the median dorsal 

 row is less prominent. Finally the ribs on the sides of the body- 

 chamber break up into a number of large nodular tubercles. 



The fossil now described is a phosphatic cast, and was found by 

 one of us (A. J. J.-B.) in the glauconitic chalk (? Bed 13) above the 

 zone of Ammonites Mantelli, in a fallen block at Humble Point, east 

 of Charton Bay, near Lyme Regis. The name pentagonus refers to 

 the pentagonal outline of the dorsal surface. It is doubtless a 

 derived fossil and is associated with A. hippocastanum, A. navicularis, 

 and Scaphites cequalis. 



Ammonites (Acanthoceeas) hippocastantjm, Sharpe, var. com- 

 presses, nov. (PI. Y. figs. 2-4 a.) 



The specimens to which we here draw attention may be regarded 

 as a variety of the above species, but so different are they in general 

 appearance that we were at first inclined to regard them as a dis- 

 tinct species. They do not, however, differ from A. hippocastanum 

 more than some varieties of A. varians do from A. Coupei, and in 

 both cases intermediate forms occur. Messrs. Sharman and Newton 

 have kindly examined the specimens and concur in this view. But a 

 form which departs so greatly from the figured type seems to merit 

 description and illustration, especially as it is by no means 

 uncommon on the Devon coast, in the layer of phosphatic nodules 

 which are often cemented to the top of the A. Mantelli-zone 

 (Bed 12), and in the overlying glauconitic chalk (Bed 13). It may 

 be mentioned that the ordinary inflated form of A. hippocastanum 

 is present in the same beds. 



For the compressed form now described the varietal name of 

 compressus is proposed. The dimensions of that figured in PL V. figs. 

 4 & 4 a are : — longest diameter 1*2 inch, lesser diameter about *88 

 (seven-eighths) of an inch, width of the mouth about -3 of an inch. 

 The whorls are broad, the sides flattened, and the back elevated. 

 About twenty tuberculated ribs pass over the back, but nearly half 

 of these die away on the sides, converging to a row of ten or eleven 

 tubercles which surround the umbilicus. 



Viewed from the back, five rows of tubercles are visible, but the 

 two outer rows are small and distant. The three inner rows are 

 set close together along the back and are laterally compressed, so 

 that the back is narrow and has a very different aspect from that of 

 the typical hippocastanum, which further differs in having large and 

 prominent lateral tubercles. Nevertheless varieties occur which 

 seem to link this form with hippocastanum, and one of them is 



