OXYNOTICERAS, MORTONICERAS, ETC. 237 



tinct from that which occurs in the old of any of the Arie- 

 tifhe. Here, as elsewhere, however, a single characteristic 

 unites the two ; the sutures are similar in both families. 

 The similarities of the young- are such as occur commonly 

 between what are supposed to be very widely separated 

 adults in many other distinct families or groups. 



Genus OXYNOTICERAS. Hyatt. Characters those of the family. 

 OXYNOTICERAS GTJIBALIANUM, d'Orb. T. Ill, figs. 653, 654. 

 To the above must be added the following genera character- 

 ized by the late Prof. F. B. Meek : 



Genus MORTONIOERAS, Meek. Shell discoid ; periphery with a 

 single, simple, low, central keel, and a more or less denned sulcus 

 on each side of it. the siilci being generally each margined exter- 

 nally by a row of compressed nodes; umbilicus wide; volutions 

 narrow, slightly embracing, and ornamented by regular, simple, 

 straight, tnberculau'd cosra 1 . 



Apparently Cretaceous only ( Cn.ifi'd States, India); whilst the 

 restricted genus Aininon il<'* is probably confined to the lower 

 members of the Jurassic system. 



A. VEspERTi.NMS, Morton = A. TEXANUS. Roemer. T. 105, 

 lig. f>3<). 



Genus PRIONOCYCLIS, Meek. Shell discoid, with more or less 

 depressed periphery having a central keel defined by a con- 

 ciVvity on each side ; keel at first simple, but at a later period 

 strongly eremite, and in old shells depressed or broken up into 

 a row of elongated nodules; volutions more or less compressed, 

 and but slightly embracing ; surface costate and tuberculate ; 

 septa with about three lateral lobes on each side, the first of 

 which is longer than the siphonal lobe and tripartite at the end, 

 while the others are much smaller and trifid, or the middle one 

 sometimes bifid; first lateral sinus broad and bilobed, the outer 

 lobe lapping partly on the peripheral side. 



AMMONITES SERRATO-CAEINATUS, Meek. Cretaceous. United 

 States. 



Subgenus PRIONOTROPIS. Meek. Shell when very young, with 

 costa.'. sharply defined, and as the whorls increase in size, becom- 

 ing more distant, without having the intervening spaces occupied 



