76 INFERIOR OOLITE AMMONITES. 



it is convenient ; and all the forms are not in a false position. As an example of 

 what I mean I may note that Lioc. apertum might be described as intermediate 

 between Lioc. v-scriptum and Lud. rudis, but it is evidently nothing of the kind ; 

 it is neither ancestor nor descendant of Lud. rudis. 



In this Table the dotted lines show that there exists some resemblance between 

 the forms which they connect ; but this resemblance is not so great as that which 

 exists between the forms joined by the unbroken lines. The most curious fact 

 here brought under our notice is that Lioc. concavum, variety formosum at the one 

 end, and Lioc. apertum at the other, are the two forms which respectively have the 

 most resemblance to certain Ludivigige, the first to Ludivigia cornu, and the second 

 to Jmdwigia rudis. But the two species of Ludwigia, whose very close relationship 

 to each other is undoubted, belong to a genetic series different from that of 

 Lioceras. This fact shows very well the extraordinary tangle which has to be 

 unravelled, and the close attention that is required, before coming to a decision 

 either as to the separation or the lumping of various forms. 



Lioceras apeetum, 8. Buchnan. Plate X, figs. 10, 11 ; Plate XV, figs. 3, 4, 7 — 10. 



Discoidal, somewhat compressed, subcarinate ; whorls flattened, with a concave 

 depression in the inner area when the inner margin is formed, and ornamented, up 

 to attaining a diameter of about fifteen lines, with simple, slightly bent ribs. After 

 this diameter has been attained the ribs gradually become more bent, less con- 

 spicuous on the inner area, and more conspicuous on the outer ; and not being 

 projected forwards on the ventral area they partake of the V-shape. The ventral 

 area is smooth, very little sloped, and ornamented with a very slight carina. The 

 inner margin is concave, and becomes more distinctly formed at about the time the 

 ribs change ; but before that period it is a mere undefined slope. The inclusion in 

 young specimens reaches to about half the preceding whorl (PL XV, fig. 7) ; in older 

 individuals to about two-thirds (PL XV, fig. 3), and in larger forms to about three- 

 fourths (PI. X, fig. 10). The umbilicus is shallow, showing portions of all the 

 inner whorls ornamented with very small ribs. 



A less compressed form (PI. XV, figs. 9 and 10) shows somewhat coarser and 

 less bent ribs, and a slightly deeper umbilicus ; but it is chiefly interesting for the 

 mouth-appendages which it exhibits. These consist of two lateral processes, evi- 

 dently of some length ; but there is no process on the ventral area, the end of the 

 mouth crossing the carina at a right angle. 



This most peculiar species has been a source of considerable trouble. At first 

 glance it seems to be intermediate between Ludivigia rudis and the young of Lioc. 



