LIOCERAS V-SCRIPTUM. C9 



inclusion, gives a peculiar flatness to the inner portion of the umbilicus in the 

 adult, and in fact in Lioceras generally. The termination shows us a small pointed 

 process on the lateral area ; but on the ventral area, where the marginal edge meets 

 the carina at right angles, there is no ventral process. 



At a diameter of 25 — 30 lines the shell has a very similar appearance. The 

 V-shaped ribs are very distinctly shown; the inner margin begins to become 

 distinct, a concave depression to exist along the inner area, and the proportionate 

 contraction of the umbilicus commences. 



The change from V-shaped to sigmoidal ribs is, of course, gradual, and takes 

 place between the diameter of four and five inches by the ribs being less projected 

 on the lateral area, and the ribs (or rather strige) more projected on the ventral 

 area, thus meeting the carina at a more and more acute angle. 



The ornamentation of the young forms of this variety is very striking ; but at 

 the same time we must not lay too much stress upon the V-shaped character of the 

 ribs. The similarity which the specimens present to Lioc. concavum in every 

 other way tend at once to show that this form of ribbing was not a character of 

 great importance; and this view is confirmed when we see that the V-shaped 

 ribbing gives place to the sigmoidal before maturity is reached. The V-shape 

 reminds us to some extent of the ribs of Ludwigia, especially Lud. obtusa ; but we 

 can see that we have no specimen of Ludwigia to deal with in the present instance, 

 both on account of the suture-line, and because on the inner area the ribs are 

 scarcely developed, while they are strong on this area in Ludwigia; moreover we 

 scarcely ever meet with any bifurcation of the V-shaped ribs, which again is the 

 rule in Ludwigia, where the strong rib on the inner area produces two smaller 

 ribs on the outer area. 



The character of the V-shaped ribs generally escapes observation. The difference 

 between Lioc. v-scriptum and Lioc. concavum is striking, but on first acquaintance 

 with the shells is very seldom appreciated. There is a greater angle on the lateral 

 area in Lioc. v-scriptum than in Lioc. concavum ; but this character varies, and there- 

 fore is not thought to be of much importance. The real difference between the two 

 styles of ribbing is only understood when the following facts are grasped, namely, 

 that in Lioc. v-scriptum the external edge has been retarded, and thus, instead of 

 the ends of the radii being projected or thrown forward on the ventral area, and 

 so causing the radii to form a kind of hook on the outer area, they are retarded and 

 cross the carina at right angles instead of joining it with a forward sweep. When 

 once this is noticed it is easily recognised, and although complicated by the V-shaped 

 changing into hook-shaped ribs, yet it materially aids in separating these forms. 

 As a consequence of this retardation of the ventral area, we find no ventral process 

 in specimens which possess the V-shaped style of ribbing. Even when such 

 specimens are adult and their V-shaped ribbing has given place to sigmoidal, the 



