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 obtusum of the same age, and the pilae 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 Ast. obtusum, stellare, or acceleration. 



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 pilse 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 

 stao-e 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 pilae. 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 Amm. obtusus, which also belongs to this species. 

 This last is beautifully preserved, and may be compared with obtusum of the same 

 age and perfection. 



The forms identified by Oppel in the Museum at Munich as Amm. Turned 

 precisely accord with the above. 



The adult specimens of one variety of Turneri and a large variety of Am. 

 ceras, found together in the Planicosta bed at Lyme Regis, are quite shnilar, and 

 have led to confusion in names, though the young are distinct, and the pilae 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, 1 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- 

 beari. One fragment of an outer whorl from Lyme Regis in the Museum of 

 Comparative Zoology is very similar to a Conybeari of the same size. It has pilae 

 not quite so straight, the geniculae form an even curve with the pilae, 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. Schwab. Jura, pi. xix. fig. 6-8 (not fig. 5). 



