122 PALEONTOLOGY. 



icus ; aperture, as determined from transverse sections of the volutions, com- 

 pressed, crescent-shaped, being very profoundly sinuous on the inner side 

 for the reception of the involuted turns; internal cast generally showing- four 

 subequidistant, transverse furrows to each turn, left by a thickening of 

 the lip within at four regular periods of cessation in the growth of the shell 

 to each vohition ; each of these furrows, or constrictions, bends a little back- 

 ward in starting from the umbilicus, and then passes nearly straight, or with 

 a slight backward or forward curve over the periphery. Surface generally 

 appearing almost smooth, or only showing obscure lines of growth, with 

 apparently sometimes the faintest possible traces of longitudinal striae on 

 the rounded periphery. 



Septa, as made out by Mr. Gabb in the California Report (none of 

 those I have seen show the septa), with siphonal lobe narrow-oblong, being 

 about twice as long as wide, deeply divided into two slender, nearly par- 

 allel, bifid, and digitate terminal branches, with two or three smaller 

 lateral branchlets on each side; first lateral lobe a little shorter than 

 the siphonal lobe, and trifid at the extremity, with two or three lateral 

 branchlets on each side; second lateral lobe slightly shorter than the first 

 lateral, and similar, excepting that it is bipartite at the extremity, with the 

 terminal divisions bifid; third lateral lobe projecting a little beyond the 

 second, and divided more nearly like the first lateral, but otherwise some- 

 what smaller. Between this and the umbihcus there, are two much smaller 

 lobes, the first of which is two or three times as large as the second, and 

 more distinctly tridigitate at the extremity. First lateral sinus smaller 

 than the siphonal lobe, longer than wide, and tripartite at the extremity, 

 with short lateral branchlets ; second lateral sinus of much the s?.me size 

 and form as the first, while the three succeeding sinuses diminish rather 

 rapidly in size toward the umbilicus, and show a more or less distinct tend- 

 ency to develop similar divisions to those of the others. 



Greatest diameter, 2.25 inches; convexity, 1.35 inches. 



Although I thought, from sketches of this shell sent to me some years back 

 for comparison by Mr. Gabb, that it probably could not be properly separated 

 from A. Ausseeanus of Hauer, a careful examination of a series of specimens 

 reveals some differences that lead me to conclude that it is more probably only 



