LIOCERAS DECIPIENS. 33 



Lioceras decipiens, var. intermedium, S. Buchman. Plate XI, figs. 2 — 7. 



Type. — Discoidal, somewhat compressed, carinate, whorls broad, slightly convex 

 on the outer and slightly concave on the inner area, ornamented with somewhat 

 indistinct sigmoidal ribs until the shell obtains the diameter of about 16 lines, and 

 then they change to very fine sigmoidal lines of growth, with a marked curve on the 

 middle of the lateral area. The ventral area slightly slopes and is sub-convex, 

 carrying a distinct but by no means trenchant carina. The inner margin is concave 

 sloping, and shallow. The inclusion is three-fourths of the preceding whorl; the 

 umbilicus quite regular, rather shallow, and ornamented with small ribs. 



The sub-variety A is very similar, but the carina is very slightly more con- 

 spicuous ; the ribs are at all times rather obscure ; the whorls do not seem to 

 increase in breadth so quickly ; the umbilicus is distinctly larger and shows 

 scarcely any ornamentation, while the inner margin is more upright. This sub- 

 variety is also more compressed than the varietal type. 



The sub-variety B is again different in many ways. The ribs are more distinct 

 and continue until the shell attains a larger diameter ; they are indistinct on the 

 inner area; the carina is similar to that possessed by the varietal type, but the 

 ventral area is somewhat flatter ; the umbilicus is distinctly larger and shows 

 more of the inner whorls, with small but fairly conspicuous ribs ; the inner margin 

 is more upright and the sides of the whorls are nearly parallel. 



These three very peculiar forms constitute a more or less connecting link 

 between the genus Hyperlioceras and the true Liocerata. The term intermedium 

 is appropriate, for they seem to possess some of the characters of three species 

 which occur in the same stratum with them. On looking at them all anyone can 

 well understand the perplexity which has so frequently accompanied attempts 

 to separate the Dorsetshire Ammonites; and we need not wonder at the often 

 expressed opinion, that hybridisation among Ammonites was a thing of common 

 occurrence. To my mind it seems that we have at present to deal with a genus 

 {Lioceras) which, having existed for some time, became one of the dominant 

 genera, and constantly made new efforts at variation. In time these variations 

 would become more and more marked, and their characters more constant, until 

 they could be fairly ranked as species. At this stage they could not be directly 

 united together by any coeval intermediate forms, their common ancestor being 

 the only link between them. 



Lioc. intermedium, as I have said, combines some of the characters of at least 

 three different species, and, curiously enough, two other genera. With the ventral 

 area, carina, and general shape of Ludwigia cornu, the suture-line and ribbing 

 of Lioceras, and the early absence of ribs, which gives them the very smooth 



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