60 PALAEONTOLOGY OF CALIFORNIA. 



what flattened ; dorsum regularly rounded, narrower than in the 

 preceding species ; umbilicus small, closed ; surface of the shell 

 sloping inwards for but a short distance. Distance between the 

 septa on the dorsum equals about half the distance between the 

 umbilicus and the dorsum. 



Septa arching gently outwards and forwards for about half 

 their length in a regular curve, which becomes more abrupt, and, 

 suddenly bending backwards, forms a long tongue or lobe, the 

 sides of which are nearly straight; from this lobe the septal margin 

 proceeds directly to the dorsum in nearly a straight line. Surface 

 smooth, or marked only by fine radiating lines, Siphon large. 



Figure, natural size, from a Martinez specimen. 



Localities: Martinez and Clayton (Div. A.), and Canada de las Uvas (Div. B.). 



Four specimens of this species have been found, of which two were collected by 

 Mr. Mathewson. A very young, but perfectly characterized specimen, was found 

 by myself at Alizos Creek, near Canada de las TJvas ; and, more recently, I was 

 fortunate enough to procure a nearly perfect specimen, sixteen and a half inches 

 in diameter, at Clayton, Contra Costa County. All of the specimens are unfor- 

 tunately too much weathered on one or the other side to give a good transverse 

 section. A perfect shell would probably present nearly the same outline as Fig. 

 3 b, except that it would be narrower on the dorsum and more regularly rounded. 



AMMONITES, Brug. 

 A. subtricarinatus, D'Orb. 



PI. 10, Fig. 4, and 4 a. 



(A. subtricarinatus, D' Orbigny . Prodrome; Vol. 2, p. 212, Etage 22. No. 9.} 



(A. tricarinatus, D'Orb. (not Poidez), Pal. Fr., p. 307, PI. 91, Fig. 1, 2.) 



Shell discoidal ; whorls many (number unknown) slightly em- 

 bracing. Section of whorls subquadrate, narrowest towards the 

 dorsum, Sides marked by a few large ribs, which rise at the 

 margin of the umbilicus, sometimes by means of a small, flat- 

 tened tubercle, pass obliquely forwards and outwards, and termi- 

 nate in a large distinct tubercle. 



Dorsum marked by three prominent angular ridges, the middle 



