58 INFERIOR OOLITE AMMONITES. 



margin, which then causes the umbilicus to appear deeper. When once the inner 

 margin is definitely formed, occlusion extends to its upper edge ; and the super- 

 position, in this manner, of the steep concave inner margins each time the whorls 

 come round tends to form a small but deep umbilicus, instead of the open one 

 which the first few whorls possessed. The total result is a ' concave ' umbilicus, 

 which appears as if it had been taken out with a gouge, and which may very 

 aptly be compared to the inside of a thimble. 



The inner whorls of a specimen with falciform ribs show us that the ribbing 

 commences at what is about the third or fourth whorl, that is, at about the time 

 when the keel begins to be distinctly formed, and when the specimen assumes 

 the flattened shape which it possesses from now onwards. The ribs do not seem to 

 begin with any appearance of the V-shape, as we might perhaps expect ; but they 

 are fairly straight, though at the same time bent forwards on the ventral, and 

 slightly curved on the lateral, area. Before the specimen attains the diameter of 

 half an inch the ribs are distinctly formed, and do not show any trace of that strong 

 reflexion which specimens of the second group exhibit ; but they have something 

 between a falciform and a sigmoidal shape. From this time onwards, the ribs or 

 radii develop a rather greater angle on the lateral area ; and, according as the inner 

 portion may be rather straight or slightly flexed, so they either approach a falci- 

 form or else a sigmoidal shape. As a matter of fact, as the inner margin becomes 

 distinctly formed, the ribs on the inner area degenerate into strias. The smooth- 

 ness of the inner area, due to this absence of ribs, is one of the generic characters. 



Among the forms in the first class we find that a slight amount of obscure 

 bifurcation of the ribs takes place about the middle of the lateral area — two ribs 

 sometimes appearing to spring from one line of growth ; but in reality this apparent 

 bifurcation is caused by the unequal size of the stria3 — the smaller being less noticed 

 beside the larger, — or else by the very close approximation of the stria3 in the more 

 finely ribbed varieties. In the type form the striae on the inner margin are fairly 

 straight, and may be likened to the handle of a sickle ; but the ribs can never be 

 said to obtain the very strong sweep on the outer area which characterises the 

 Harp.-falciferum-grouip. Nevertheless, the forward sweep on the ventral area 

 distinguishes the typical forms from those of the second class, in which the ribs 

 meet the ventral area at right angles — or sometimes even point slightly backwards — 

 and, continued as striae, cross the carina at about right angles. The Y-shaped radii 

 of this second class gradually change to sigmoidal or sub-falciform ribs ; and this 

 change takes place at a diameter varying from rather over one inch to four or five 

 inches according to the variety. 



Whether or not the typical forms possessed, in the young state, a pointed lateral 

 process, as in Lioc. v-scriptum, I cannot say ; but it is probable that they did, though 

 the straighter flexures of the ribs would indicate that it was not very important. 



