MUSEUM OF COMPARATIVE ZOOLOGY. 73 



Psiloceras planorbis Hyatt. 



A mm. planorbis Sow., Min. Conch., v. 5, p. C9, pi. 448. 

 Loc. Semur ; Coll. Boucault. 



Psiloceras planilaterale Hyatt. 

 Loc Semur; Coll. Boucault. 



Sides flattened, but more convex than other species, and marked with 

 transverse striae ; occasionally plicated at intervals, resembling in this 

 respect plicated variety of P. psilonolus. Abdomen depressed, convex, 

 smooth ; the siphon merely indie ited by a raised line in the adult. 

 Umbilicus shallow, showing great breadth in the young. 



Psiloceras acutidorsale Hyatt. 

 Loc. Semur ; Coll. Boucault. 



Closely allied to P. psilonotus, but with smaller whorls and a more 

 acute abdomen. It is, also, not so gibbous, and has a greater number of 

 whorls than either P. planllateralls or P. planorbis. The shell may be 

 marked with very numerous plications, or smooth on the sides. Abdomen 

 prominent, acute. Umbilicus shallow. 



Note. It i> probable that Amm. lalesulcatus Hauer, Ueber d. Ceph. aus d. Lias d. 

 Nordustlichen Alpen, p. 44, pi. 9, figs. 1 - 3, is the type of another genus of this family, 

 having a keeled and sulcated abdomen. 



DISCOCEEATID M* 

 Arniocerast Agassiz. 



Abdomen keeled and channelled, but both parts are variable, being 

 sharply defined in some species and very shallow in others. Abdominal 

 lobe shallow and broad ; not so deep as the superior lateral lobe ; deeper 

 than the inferior lateral ; both divided equally. Superior lateral cell 

 equally divided. Inferior lateral cell unequally divided. The young 

 retain the smooth character for some time during their growth, thus giving 

 to the umbilicus a decidedly embryonic aspect. Envelopment extends 

 laterally to the genicular. 



Arnioceras cuneiforme Hyatt. 

 Loc. Semur; Coll. Boucault. 



Sides regularly convex. Pila; depressed, most prominent in the centre, 

 and sloping gradually to either side ; striae of growth very fine and sharply 

 bent. Abdomen obtusely angular; keel indicated by a ridge; channels 

 obsolete or only indicated by shallow depressions. The auxiliary lobes 

 near the umbilicus are hardly more than mere serrations. 



* Equals Arietes. f Apvews, a ram. 



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