FIFTH, OR AGASSICERAN BRANCH. 209 
fold-like character common in those species. They are more acute and about 
equally prominent near the dorsum, and also on the edge of the abdomen, and 
often interrupt the channel ridges. The earliest period examined was the third 
quarter of the fifth whorl. One specimen, from Lyme Regis, had a much nar- 
rower channel area than in obfusum of the same age, and the pile reached nearly 
to the base of the keel; in another, from Gloucester, the channels were better 
developed, but extremely narrow. This species even on the fifth volution had a 
better developed keel, deeper and more distinctly marked channels, and flatter 
sides, than any variety of As/. obtusum, stellare, or acceleratum. 
One specimen from Semur on the second quarter of the sixth whorl had 
lobes differing from two fifths to three fifths, and saddles from two fifths to. 
one half. 
Senile characteristics begin to appear on the latter part of the seventh whorl. 
The pile diminish to large folds on the second quarter of the eighth whorl, and 
subsequently disappear altogether. The channels also increase in breadth and 
diminish in depth. The keel acquires greater prominence in the clinologic 
stage on account of the shallowness of the channels, but finally becomes 
depressed. The largest specimen measured had about eight whorls, and the 
diameter was 327 mm. 
A fine series of this species is in the Museum of Stuttgardt. These shells 
became smooth at variable ages. One from Balingen, about 113 mm. in diam- 
eter, had become entirely smooth; another from Endigen nearly half a whorl 
older had still very prominent pile. The peculiar form of the whorl, the deep 
channels and flat sides, hold constantly, however, in these, as well as in a young 
specimen from Balingen, labelled Amu. obtusus, which also belongs to this species, 
This last is beautifully preserved, and may be compared with obéuswm of the same 
age and perfection. 
The forms identified by Oppel in the Museum at Munich as Amn. Turnert 
precisely accord with the above. 
The adult specimens of one variety of Turneri and a large variety of Arn. 
ceras, found together in the Planicosta bed at Lyme Regis, are quite similar, and 
have led to confusion in names, though the young are distinct, and the pile of 
Arnioceras are not bent or curved as in Ast. Turneri. The same error has been 
also repeated by Quenstedt in his identification of Amm. cf. obtusus,' a variety 
of ceras from Ofterdingen, with the English. form of this species as figured by 
Wright, i. e. with the young as shown in Wright’s plate. 
Another source of confusion lies in the resemblances of fragments of the 
more discoidal variety of this species, when of considerable size, to Ver, Cony- 
bear’. One fragment of an outer whorl from Lyme Regis in the Museum of 
Comparative Zoilogy is very similar to a Conybeari of the same size. It has pile 
not quite so straight, the genicule form an even curve with the pile, and the 
abdomino-dorsal diameter is greater in proportion than is common in that species. 
The abdomen is precisely similar in its keel and channels. 
1 Amm, Schwib. Jura, pl. xix. fig. 6-8 (not fig. 5). 
27 
