18 PROCEEDINGS OF THE NATIONAL, MUSEUM vol. 78 



the presence of several additional rows of nodes on the flanks of 

 Pseudaco7n.'psoceras and greater curvature of the ribs. 



Hyatt placed Acompsoceras in the family Mantelliceratidse. Per- 

 vinquiere placed it between Acanthoceras {Sharpeiceras) and Mam- 

 mites, in his discussion suggesting that both Acompsoceras and Mam- 

 mites were derived from Acanthoceras. Spath assigned both Acomp- 

 soceras and Pseudacompsoceras tentatively to the Schloenbachiidse. 



The genus may be characterized as follows : 



Discoidal ammonites with moderately wide umbilicus ; large, fold- 

 like costae ; marginal and umbilical rows of nodes ; venter flattened or 

 somewhat convex, bearing a raised line or " nascent keel." In later 

 stages the flanks became smooth and the venter rounded, the nodes 

 disappear. The external suture has five or six lobes and shows a 

 curvature with the second lateral saddle high; saddles bifid, lobes 

 bifid; siphonal lobe notably shorter than the first lateral lobe. 



ACOMPSOCERAS AMERICANUM, new species 



Plate 3, figures 1-4 



This species is based on a single internal mold, a fragment consti- 

 tuting a little more than one-fourth whorl, accidentally compressed 

 and somewhat distorted but preserving essential characters suffi- 

 ciently well to deserve a definite assignment and a specific name., 



Shell apparently a compressed disk; maximum diameter of type 

 now remaining estimated at 75 millimeters, with part of living 

 chamber preserved. Umbilical shoulder rounded ; width of umbilicus 

 not known, but evidently small. Aperture unknown. 



Sculpture on septate part of the type consists of fairly sharp 

 primary ribs forking to form low rounded secondary ribs, between 

 which lie intercalated secondary ribs. All these end at the ventral 

 margin in elongated transverse nodes. No umbilical nodes are visible 

 on type. The unseptate part shows only faint growth lines. The 

 ribs and growth lines are gently falcate. The venter was probably 

 gently convex with a low but distinct raised line, rounded on the 

 internal mold. The width of the venter at the maximum diameter of 

 the type (estimated at 75 millimeters) is 9 millimeters, but it may 

 have been reduced a little by the distortion of the shell. 



The suture shows bifid lobes and saddles, moderately dissected, 

 short siphonal lobe ; first lateral saddle and first lateral lobe subequal 

 in width; second lateral saddle high. 



Acompsoceras americanum is characterized by its numerous ribs 

 on the early part, its lack of distinct umbilical nodes, and the lesser 

 dissection of the suture as compared with other species. From A. 

 hochumense (Schliiter) it differs in its more numerous ribs, lack of 



