13 
later Callovian species may be an accident of preservation and deposition. 
The stratum in which a fossil occurs is not always true evidence as to its 
date. 
Five micromorph specimens now remain. That they belong to the 
genera Paracephalites or Metacephalites cannot be accepted: they differ 
both in ornament and suture-line, though the details as to the suture-line 
are unsatisfactory, partly owing to condition of fossilization, partly owing 
to the fact that in several specimens the preserved part of the last whorl 
is body-chamber. 
The character of the ribbing is the reason for separating the micro- 
morphs from the other genera: the ribs are elevated and compressed, that 
is to say, that in cross-section they are lath-like or in the form of laminae, 
they pass over a venter of somewhat flattened convexity with what shows 
as a slight forward sweep, yet on the actual periphery itself the ribs are 
almost straight — the appearance of curvature is due to the slight forward 
bending of the ribs on the outer lateral area as they approach the venter. 
The ribs rise in sharp waves or transverse crests on the venter, like a series 
of slightly bent laths placed crosswise. 
These characters are not seen in the other genera and do not belong 
so far as is known to genuine Macrocephalites: but similar characters are 
depicted by Waagen in several Macrocephalitoids, for instance in his 
Stephanoceras subtrapezinum (PL XXXIII, f. 4) from the Golden Oolite, 
macrocephalus zone. 
On the evidence of certain Macrocephalitids from English rocks, 
which are before me, showing a passage from ribs straight across venter to 
ribs curving across venter as the shell grows older, it may be inferred that 
curved ventral ribs are not a primary but a secondary character in Macro- 
cephalitidae. 
In the Tulitidae curved ventral ribs are a primary character, already 
established in the cadicone state: in the Macrocephalitidae curved ventral 
ribs may not appear at all, but if they do it is not until long past the cadi- 
cone stage: they may appear when the contraction leading to platycone 
stage is beginning or may not appear until it is well advanced. This differ- 
ence in relative development of characters between Tulitidae and Macro- 
cephalitidae is important and if curved ventral ribs are a feature of fairly 
late development in Macrocephalitidae, are a secondary not a primary 
feature, it indicates that the forms that possess curved ribs when quite 
small may be regarded as biologically somewhat late Macrocephaltid 
developments. These small forms are not to be regarded as the young of 
larger examples, but are mieromorphs, possibly catamorphs of the Macro- 
cephalitidae. That they possess in most cases nearly complete body- 
chambers fits in here. 
As curved ventral ribs are a later development of straight cost®, so 
by the law of cyclical development — the tendency in old age to return to 
characters of youth — renewed development of straight ribs after curved 
may be expected. This is shown to a small extent in Metacephalites. 
77866 — 2i 
