42 



GENUS ANCYLOCERAS, D'0r&. 

 ANCYLOCERAS JENNEYI, n. sp. 



Plate 16, figs. 6-8. 



Shell ot moderate size, composed of two or more(t) rapidly-increasing 

 volutions, enrolled on the same plane, and entirely disconnected ; the 

 inner ones apparently leaving a broad, open umbilical space. Shell 

 rather rapidly increasing in size with increased age. Volutions slightly 

 flattened, on the back vertically compressed and transversely oval in 

 section j the vertical diameter being only about two-thirds as great as 

 the dorso-ventral in the larger part of the shell, the greatest height 

 being nearest to the ventral margin. 



Surface of the shell marked by strong, distant, transverse, encircling 

 ridges, which are directed slightly forward in passing from the dorsal to 

 the ventral margin, and in crossing the latter they become nearly obso- 

 lete. Occasional intercalated ridges occur at irregular distances on the 

 dorsal side. Intercostal spaces concave. The shell is further orna- 

 mented by two lines of obtusely rounded nodes, which are situated on 

 the ribs at the angles of the flattened dorsum. 



Septa somewhat complicated and closely arranged, sothat the inner 

 lobes interfere with the extremities of the sinuses of the one in advance but 

 on the dorsal portion they are more distant. Siphonal lobe longer than 

 wide, deeply divided at the extremity and terminating in two large, strong- 

 ly serrated digitatlons, having one or two lateral spurs on their outer side. 

 First lateral lobe large, deeply bipartite, each branch terminating in 

 two or three unequal digitatious, and with several smaller spurs on the 

 sides above. Second lateral lobe much smaller than the first, deeply 

 bipartite, with numerous strong spurs irregularly arranged along the 

 margins. Antisiphonal lobe long, narrow and simple, but with long 

 lateral serrated spurs and a slightly tripartite extremity. First and 

 second sinuosities rather large, deeply divided by large auxiliary lobes 

 into two principal branches, which are again deeply divided. Third 

 sinus smaller than the others and divided into two principal branches. 

 The margins of all lobes and sinuses deeply serrated. 



The species is described from an imperfect individual, and a second 

 smaller fragment of a larger specimen. They consist only of the con- 

 volute parts, and show no evidence of the deflection of the tube. It is 

 possible they are not rightly referred to the genus Ancyloceras, but are 

 more nearly related to the genus Anisoceras of Pictet, which differs 

 from the former genus in being irregularly helicoid. The example fig- 

 ured would seem to have been somewhat irregularly coiled, but this 

 may possibly be the effects of distortion, as the smaller coils of the shell 

 are considerably displaced. It differs from Ancyloceras also, as defined 

 by Mr. F. B. Meek (Pal. L T . S. Geol. Surv. Territ., p. 409), iu having 



