132 PALAEONTOLOGY OF CALIFORNIA. 



being exactly as wide transversely, as from tlie dorsum to the umbilical margin. 

 The periodical constrictions on the cast figured, are about as numerous on these 

 specimens as on the flatter forms, the thickening of the shell at those points show- 

 ing itself as a faint rib on the outside; a specimen before me has eight on one 

 volution. The other ribs, mentioned in the description and shown in the figure 

 quoted above, are entirely absent in some cases. No differences exist in the septum. 



A. Batesii, Trask. 



PI. 20, Fig. 9, a; PI. 21, Fig. 10, a, b. 



(A. Batesii, Trask; Proc. Cal. Acad., 1855, p. 40.) 



(Id., Gabb ; Pal. Cal., Vol. 1, p. 67, pi. 13, fig. 16, 16 a, b.) 



The largest known, most widely diffused, and one of the most variable Ammon- 

 ites of California. 



i 



The form described by Dr. Trask, and used by me as the typical form of the 

 species, is many whorled, the volutions nearly circular in section, increasing very 

 slowly in size, and barely more than in contact. In this form the surface is usually 

 ornamented by simple, linear ribs, with fine revolving lines in the interspaces. 

 Another variety was mentioned, in which the whorls increase much more rapidly 

 in size, several specimens having been found of over a foot in diameter. A third 

 variety has since been obtained, in which, in the young state, up to an inch, or an 

 inch and a half in diameter, the whorls are broader than high, the dorsum broadly 

 rounded, and the umbilicus occupies more than half the diameter of the shell, is 

 funnel-shaped, very deep, and is bordered by a sharp angle, the surface between 

 this angle and the suture being flat. As the shell in this variety grows older the 

 angle disappears, the flattening gradually rounds out, and the older shell assumes 

 the normal form of the species, except that it bears an occasional well-marked, 

 rounded, slightly sinuous rib ; the linear ribs are found in the very young shell, 

 but have not been detected in the older forms of this variety. In all of these 

 varieties the septum, which is very characteristic of the species, continues the same. 

 The figure of the septum given in the first volume, is from a weathered specimen, 

 and has lost all of its more minute details. Besides the parts figured and described, 

 the ventral lobe seems to be very large, and projects a spur beyond the suture, the 

 points reaching to and intruding between the tips of the lower branch of the 

 inferior lateral. 



A. Tehamaensis, Gabb. 



(A. subtricarinatus, Gabb (not d'Orb.) ; Pal. Cal., Vol. 1, p. 60, pi. 10, fig. 4.) 



Further study of this shell leads me to believe that it is distinct from subtricar- 

 inatus. It differs from that species in having three distinct, acute dorsal keels; 



