2l6 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Hyatt (1889) thought that Neumayr's genus was founded 

 on immature specimens, probably of the genus Agassi zee?' as, 

 while Haug does not think that the characters upon which 

 the genus was based are sufficiently constant to warrant the 

 separation. But whether Cymbites is a valid genus or not, 

 there must have been such a form in the ancestral line of the 

 ^goceratidcC, and thus of Hoplites and Placenticeras. So 

 the name will be used until the genus is given a name that 

 will stand, in case the old designation is thrown out. 



Cymbites has been found only in the Lower Jura, but 

 from its character must have occurred also in the Trias, for 

 it is simpler in septa than any other ammonite of the Jura. 

 This stage lasts in Placenticeras about three-eighths of a 

 coil and ends at diameter 2.20 mm., one and five-eighths of 

 a coil from the protoconch. 



yEgoceran Stage. — With the beginning of this part of the 

 adolescent stage the division of the lateral lobe becomes 

 more pronounced, and a secondary indentation begins on 

 the side away from the abdomen. The whorl grows higher, 

 and the rounded exterior gives place to somewhat flattened 

 sides with abdominal shoulders. The sculpture becomes 

 stronger, and the shell is already decidedly asgoceran in 

 appearance. The specimen figured on PI. XXIV, figs. 12 

 and 13, and the septa on PL XXVII, figs. 6-8, show three 

 internal lateral lobes, an advance in development that is 

 more forward than that of Hoplites diifrenoyi at diameter 

 5.00 mm. At one and seven-eighths coils the young ammon- 

 ite gave the following dimensions (PL XXIV, figs. 12 and 



13):— 



mm. 



Diameter 2 . 64 



Height of last whorl 1.19 



Height of last whorl from the preceding 0.98 



Width of last coil i . 16 



Involution 0.21 



Width of umbilicus o. 80 



Perisj[)}mictoid Stage. — At two revolutions, diameter 

 3.00 mm., the sides become still more flattened and the 

 abdominal shoulders squarer, giving a perisphinctoid 

 aspect, heightened by the frequent varices and the strong 



