CRETACEOUS FOSSILS. 457 



mences at the suture between the two limbs, and continues over the outer 

 surface of both, retaining- the form of the shell, surface characters, and lines 

 of nodes throughout its entire substance. On the side of the specimen seen 

 in the figure the shell is mostly removed by exfoliation, but on the oppo- 

 site surface it is retained in great part, and shows a thickness of about a 

 twelfth of an inch at the smaller end, and less than a sixteenth of an inch 

 at the larger extremity of the specimen. If the thickening had been from 

 the inside, it would have affected the inside of the tube and the parts along 

 the junction of the two limbs; but this is seen not to be the case when the 

 shell is broken. From this feature of the external thickening of the shell 

 we have thought it possible that it was an internal appendage of the animal, 

 like that of Spirula, and that it probably became enveloped within the sub- 

 stance of the mantle at the period at which the bending or folding of the 

 shell took place, even though it may have been external during the earlier 

 stages of growth. 



PTTCHOCEEAS MEEKANUM. 



Ptychoceras Meekanum Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 44. 



Shell of small size, subcylindrical or vertically compressed, giving a 

 somewhat oblate-transverse section, very gradually increasing in diameter 

 with the increased length ; smaller liinb slender, gently curved or bent in 

 the middle of its length, so as to give the earlier and middle portions of the 

 shell an angle of about 135° to each other. Larger limb, or outer chamber, 

 closely appressed and slightly embracing the smaller, and extending to near 

 the middle of the curvature. 



Surface of the entire shell marked by strong, simple, sharply angular, 

 oblique, encircling ridges, with wider, deeply-concave interspaces, very 

 regularly increasing in distance from the apex outward ; and on the dorsal 

 region by two longitudinal rows of spines or spiniform tubercles, situated 

 at about or nearly one-sixth of the circumference of the tube from each 

 other, and on the crests of the ridges. The encircling ridges are directed 

 obliquety forward in passing from the ventral to the dorsal surface in the 



