124 PALEONTOLOGY. 



'THYSANOID^. 



''Genus ACROCHORDICERAS, Hyatt. 



" {aKpoxopdcov, a wart ; Kspag, a horn.) 



"This genus is closely allied to Lytoceras and Phi/Uoceras, Suess, and 

 Haploceras of Zittel, combining characteristics which are found in all of 

 these, besides having peculiar characters of its own, and a different develop- 

 ment. The extent of involution is comparable with that of Haploceras^ but 

 the whorl itself is about intermediate between the extreme roundness of 

 Lytoceras, and the more flattened sides of Phylloceras. 



"Its peculiar characteristics consist in having large lateral tubercles and 

 abdominal pilse, which are united as they near the tubercles. The smooth 

 zone along the center of the abdomen in the young is also probably of gen- 

 eric value." — (A. H.) 



ACROCHORDICERAS Hyatti, Meek. 

 Plate 11, figs. 5 and 5 a. 



Shell discoid, with the periphery rounded; volutions rounded or very 

 nearly so, and increasing gradually in size, with each of the inner ones 

 about three-fourths embraced by the succeeding larger — all, so far as 

 known, rounded on the outer side ; umbilicus more than half as wide as the 

 dorso-ventral diameter of the outer volution, rather deep, and exposing 

 about one-fourth the breadth of each inner whorl. Surface at first in the 

 young shell nearly or quite smooth, then ornamented with small, regular, 

 straight, moderately distinct costse, that seem not to cross the periphery, 

 and die out before reaching the umbilical side, while at a later stage of growth 

 they become quite strongly developed, especially in crossing the periphery, 

 on which are intercalated occasional intermediate ones of equal size. These 

 coalesce with the others on the sides of the volutions, and give origin at the 

 points of junction to prominent nodes arranged at regularly-increasing inter- 

 vals, so as to form a single row near or within the middle of each side. 

 Protected parts also show rather distinct lines of growth, running parallel 

 to. the costse, which latter are slightly sigmoid on the sides of the larger 

 volutions, but pass straight across the periphery. 



