CRETACEOUS FOSSILS. 133 



d'Orbigny's shell possessing but one in the middle, the undulated dorsal ends of 

 the lateral ribs making the other two. The Californian species bears a series of 

 ribs on the sides, all larger in proportion than in the French shell, and entire, 

 while in the other they are alternately bifurcate ; that is to say, the ribs are alter 

 nately an entire one, reaching from the umbilical to the dorsal margin, and a Y 

 shaped rib dividing on the middle of the shell, and sending two branches to the 

 dorsum. In size, and in proportionate size of volutions, the two shells are closely 

 allied, as well as in general style of ornament; and so far as we yet know, the 

 geological horizon of the Californian is very near that of the European species. 

 The absolute age of the Cretaceous strata of the northern part of the State has 

 not yet been fully determined ; but, as will be explained in the proper place, it 

 seems not improbable that representatives of both the Gault and Lower Green 

 Sand will be found included in what has here been provisionally termed the Shasta 

 Group. It is apparently from this member that the present species is derived. 



A. SUCIAENSIS, Meek. 



PI. 21, Fig. 11, a, b. 

 (A. complexus var. Suciaensis, Meek ; Proc. Phil. Acad., 1861, p. 317.) 



A very characteristic, but not very perfect specimen of this species was found 

 near Monte Diablo. Mr. Meek, to whom I sent sketches, says that he considers 

 it identical with the Sucia Island form ; differing in some of the details of the 

 septum, but not sufficiently to indicate a specific difference. 



A. JUGALIS, D. 8. 

 PI. 22, Fig. 12, a, b', 13 a. 



SHELL small, rounded discoidal, whorls rapidly increasing in 

 size and deeply embracing; body whorl sub-appressed oil the 

 sides or, in some cases, regularly arched, dorsum rounded; um 

 bilicus small, deep, margin rounded, o sub-angulated. Surface 

 polished, or marked only by a few faint, sinuous lines, arching 

 forwards, and more distinct on the dorsum ; besides these there is 

 a variable number of thickened ribs in the substance of the shell, 

 starting from the umbilical margin, and curving outwards and 

 forwards sinuously, and crossing the dorsum, where they are 

 most strongly marked; these ribs, from three to six in number 

 on a single volution, are more prominent on the internal than on 



