40 



Volutions about six, increasing* slowly in size, the sides of the 

 inner whorls being- almost completely exposed; outer whorl nearly 

 circular, but a little compressed at the sides, its base being shallowly 

 emarginate by the preceding volution; aperture slightly wider than 

 high. 



Surface ornamented with simple, transverse, flexuous ribs, and oblique, 

 periodic grooves or constrictions. As the direction of the constrictions 

 is d liferent t© that of the ribs, some of the latter are almost always 

 truncated by the former. In the outer whorl of the specimen figured 

 there are nine or ten constrictions, each of which is narrow on the 

 periphery and wide on the inner half, at least, of the sides. They divide 

 the ribs into sets in a somewhat uniform way. Near the aperture there 

 are generally six ribs between each pair of grooves. Of these, the first 

 three are continuous, as is also the sixth, or outer one, which is so 

 oblique as to truncate about one-half of the fifth, near the middle of the 

 sides, and the base of the fourth, near the sutures. Or, it might be said 

 with equal propriety, that the outer rib trifurcates at unequal distances, 

 before passing over the periphery. In each set of ribs, the two outer 

 ones form part of the boundaries of the grooves by which they are 

 separated.. Both ribs and constrictions become faint and nearly obsolete 

 on the periphery, especially the former. The constrictions are obviously 

 the remains of former lips, which were sinuous at the sides, and produced 

 into narrowly rounded, or spout-like processes, on the siphonal edge of 

 the shell. 



Septation unknown. 



G-reatest diameter of the largest example, nineteen lines; width of 

 umbilicus, from sutui'e to suture, not quite nine lines. 



The description applies solely to the ^specimen figured, which is the 

 largest of the only two collected. The other one, which measures 

 scarcely an inch across, is comparatively flat and thin, its aperture 

 is much higher than wide, and the ribs and constrictions are not so 

 clearly defined as they become at a more advanced stage of growth. 



There is a considerable resemblance between these shells, which are 

 obviously very immature, and the A. Seranonis of D'Orbigny,* a French 

 Lower Neocomiau fossil. D'Orbignj-'s species is represented as having 

 a thinner and flatter shell than is that of A. Laperousianus at the same 

 age. Young specimens of the latter are indeed nearly as flat as the 

 Fi-ench fossil, but the umbilicus of the smallest of the two individuals 



* " Palfiontologie FranQaise. Terrains Cretaces." Vol. I., page 361, Atlas, Plate CIX;, figs. 4 and 6, 



