FOURTH, OR CORONICERAN BRANCH. 163 
In other young specimens, the acuteness of the abdomen begins upon the latter 
part of the third or first part of the fourth whorl, and the strie of growth are 
regular and well marked. Previous to this the abdomen is rounded, as in 
Psil. planorbe. 
The smoothness of the sides of the whorls, the immature folds, and the flat 
discoidal aspect of the young, make the shell very like planorbe, var. leve, and 
in the next stage the folds give an aspect somewhat similar to the plicatus 
variety of planorbe. The prolonged smooth stage of the young, before it 
takes on the folds, has no correspondence with any form of Caloceras, and 
indicates direct derivation from planorbe, var. deve. In the typical specimens 
of Arn. miserabile, var. acutidorsale, the folds are sometimes not apparent upon 
the cast, even upon the fifth whorl, but in one specimen a careful exami- 
nation showed that the original shell must have had faintly marked folds, 
which stretched entirely across the side and had the usual abrupt terminations. 
Quenstedt* figures this variety. The aperture is shown in his Fig. 27 to have 
been similar to that of planorbe, having a well marked rostrum, broad lateral 
sinuses, and a constriction. It is by no means certain, as Quenstedt states in 
the same work on page 104, that Wright’s figure of the young of semicostatum, 
Plate I. Fig. 7 of his “ Lias Ammonites,’ is a specimen of this species; it is 
quite as likely that Wright was correct. Professor Quenstedt’s specimens at 
Tiibingen are for the most part young from the Oelschiefer, but a large one? 
was nearly, if not quite, full grown. The keel in this appeared as a sharp 
ridge at an early age, and maintained the same character in adults. 
The abdomen does not broaden out as in adult of acutidorsale, but per- 
sists mM maintaining its angular character throughout life. The pile began 
quite early, but never appeared to get beyond the fold-like stage. Some- 
times, however, they bend forwards and may cross the abdomen, and then the 
abdominal ridge forming the keel is crenulated? The variety occurs in South 
Germany, especially in the Oelschiefer of Quenstedt. 
A form doubtfully referred to this variety was collected by Professor Orton 
at Ipishguaniina in Northern Peru.* 
Var. cuneiforme. 
Plate II. Fig. 7. 
Arnioceras cuneiforme, Hyatt, Bull. Mus. Comp. Zodl., I., No. 5, p. 78. 
Abdomen acute, as in variety acutidorsale ; sides regularly convex; pilx 
depressed, most prominent in the centre, and sloping gradually to either side. 
The abdominal lobe is somewhat longer than the two lateral lobes, which 
are of about equal length. Superior lateral lobes and saddles are pointed, the 
inferior lateral lobes and saddles mere serrations. 
1 Amm. Schwab. Jura, pl. xiii. fig. 27-30. 2 No. 7742 of his collection, locality unknown. 
8 This does not indicate any affinity with other genera; it is a sporadic and purely pathological modifi- 
cation, as it is also in the young of Oxyn. Oxynotum, and many other forms in which the young shells are 
often affected by similar abdominal crenulations. 
4 Proc. Bost. Soc. Nat. Hist., XVII., 1875, p. 367. 
ee 
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