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 strise 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. leve. In the typical specimens 

 of Am. 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 1 figures this variety. The aperture is shown in his Fig. 27 to have 

 been similar to that of plano?-be, 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 

 Tubingen are for the most part young from the Oelschiefer, but a large one 2 

 was nearly, if not quite, full grown. Tbe 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 in maintaining its angular character throughout life. The pilae 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. 3 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. 4 



Var. cuneiforme. 



Plate II. Fig. 7. 

 Arnioceras cuneiforme, Hyatt, Bull. Mus. Corap. ZotiL, I., No. 5, p. 73. 



Abdomen acute, as in variety acutidorsale; sides regularly convex; pilae 

 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, pi. 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. 



i Proc. Bost. Soc. Nat. Hist., XVII., 1875, p. 307. 



