142 PALAEONTOLOGY OF CALIFORNIA. 



have increased more rapidly in diameter than the others, is not 

 perfectly straight, but bends a little outwards, giving a convex 

 line longitudinally on its ventral face. Surface of the spiral por 

 tion ornamented by oblique, nodulated ribs; these are small, 

 linear, and arched forwards on the ventral face, large on the sides 

 and dorsum, and more advanced on the lower than upper side, 

 being oblique on the dorsum; they carry three pairs of nodes, one 

 on each side of the median dorsal line, one on each dorso-lateral 

 margin, and one on the middle of each side. The straight limb 

 retains these ribs, which here lose their obliquity, but retain all 

 the other characters; in addition, on this part of the shell, be 

 tween each pair of these ribs, is placed a smaller, simple rib. 

 On the return limb, the ribs lose all their tubercles, become even 

 in size, acute, and with concave interspaces; the section of this 

 portion of the shell is rounded subquadrate, a little narrower on 

 the dorsal than on the ventral side. 



Septum composed of a dorsal and ventral lobe, and two laterals 

 on each side. Dorsal lobe broad, divided for half its length, each 

 branch carrying three equal spurs on the outer side; above this, 

 on the body of the lobe, are one large and three smaller spurs. 

 Dorsal saddle divided into two bifurcate branches. Superior 

 lateral lobe longer than the dorsal, broad, and carrying three 

 slender, nearly equal branches; the middle one trifid on the end, 

 the upper lateral much more complex than the lower; above 

 these, on the body of the lobe, are two or three spurs, diminish 

 ing regularly upwards. Lateral saddle like the dorsal, but a little 

 smaller, and obliquely divided. Inferior lateral lobe not more 

 than half as long as the superior, slender and equally trifurcate. 

 Ventral saddle like the others, but smaller. Ventral lobe nearly 

 as long as the dorsal, slender, trifurcate, each bradch trifid, the 

 laterals bearing two spurs above on the outer side; the body of 

 the lobe carries two spurs above the origin of the branches. 



Total length, 6 inches; diameter of spiral, 2 inches; diameter of volution at 

 termination of spiral, .6 inch ; height of apex, .9 inch ; length of longest fragment 

 of first straight limb, 4. inches ; length of longest fragment of return limb, 2.7 



