PRONORITES. 45 



The next stage corresponds to the supposed genus Ihergiceras Kar- 

 pinskv, of which Gon. tetragonus Roemer is the type; iutliis the whoi'ls are 

 broad, low, and only slightly embracing, the nud)ilicus wide and shallow. 

 The sntures consist of a long rather narrow si})honal lobe and two broadly 

 rounded lateral lobes. This is the nepionic or larval stage (PI. XIII, 

 fig. 5«). In the continuation of tliis stage the whorls become higher and 

 the lobes more complicated, corresponding to the genus Prokcanites, of 

 which Gon. henslowi Phillips and Gon. serpentinus Phillips are typical forms. 



In the next stage the shape of shell does not change materially, but the 

 siphonal lobe becomes three-pointed (PI. XIII, fig. 5); this is the neauic 

 or youthful stage, and corresponds to the geiuis FaiaproJccanites Karpinsky, 

 of which the type is Goti. mixolobus Sandberger (not Phillips)." 



The further development consists in the division of the first lateral 

 lobe by a secondary saddle ; the shell is then in the ephebic or adult stage, 

 and in Pronorites gets no higher in its development. 



The sutures are then constant in shape, and consist of a three-pointed 

 siphonal lobe, a first lateral lobe deeply divided by a secondary saddle and 

 five secondary lateral lobes outside the umbilical border, and one on the 

 umbilical shoulder. All the lobes are pointed, and the saddles rounded. 

 The inner lobes, covered by the involution, are unknown. 



The sutures, as figured on PI. XII, fig. 15, show some differences 

 from those figured by Phillips, PI. XII, fig. 3, and by Karpinsky, PI. XIII, 

 fig. 5. On the Arkansas specimens the three-pointed siphonal lobe is 

 longer than on the type of Phillips, or the form P. cydolobus variety 

 uralensis Karpinsky, the secondary sinus on the first lateral lobe is deeper, 

 and the second lateral lobe is pi'opoi'tionally longer. In this the Arkansas 

 specimen does not depart further from the type than the variety uralensis. 

 This difference was thought to be of sufficient importance to characterize 

 a new variety, and the name P. cydolobus Phillips, variety arliansasensis was 

 proposed in 1896. 



Surface markings. — The shell is smooth and devoid of constrictions or 

 other ornamentation, ])ut on the body chamber of the adult Karpinsky 

 observed weak ribs that are stronger on the abdomen and grow weaker 

 toward the umbilicus. 



" Verstein. Rhein. Schichtensystems in Nassau, p. 67, PI. III. fii;. 1.3?; PI. IX, fig. 6. 



