26 PALAEONTOLOGY OF CALIFORNIA. 



A. HOMFRAYI, 11. 8. 

 PL 4, Figs. 18, 19. 



SHELL small, discoidal, deeply embracing; sides flattened, curv 

 ing inwards towards the dorsal margin ; umbilicus small, less than 

 a fourth of the diameter of the shell ; margin angular ; dorsum 

 bicarinate, channelled between the carinse. Surface ornamented 

 by numerous flexuous ribs, arising on the margin of the umbili 

 cus, and crossing the sides in three broad curves, bending ab 

 ruptly forwards when near the dorsum, and ending on the dorsal 

 carina, which presents an undulated or tuberculated appearance. 

 These ribs bear a variable number of rows of tubercles, the most 

 constant of which are those nearest the dorsum and on the um 

 bilical margin. Septum composed of a dorsal and three lateral 

 lobes. The dorsal lobe is divided for about a third of its length, 

 each branch bearing a couple of spurs, and with three or four 

 simple tooth-like processes above ; dorsal saddle narrow above 

 and slightly notched at the end ; superior lateral lobe broad above, 

 narrow below, trifid at the extremity, and with three or four 

 spurs above ; lateral saddle shaped almost like two sides of an 

 equilateral triangle, and slightly notched along the sides ; inferior 

 lateral lobe half the size of the superior lateral, with three irregu 

 lar spurs at the end, and slightly toothed above ; supplementary 

 lobe much smaller, but nearly a miniature of the last. 



Figures, natural size. Fig. b, septum. 



Locality: E. Eange, Humboldt, Nevada Territory. Collected by Mr. Richard 

 Homfray, to whom the species is dedicated. 



Some specimens, from the same locality, smaller than the one figured, are more 

 convex on the sides, and with the ribs proportionally a little larger and less nume 

 rous ; and with several rows of tubercles, in a few cases nearly as prominent as 

 the ribs themselves. 



A. BlLLINGSIANUS, n. 8. 

 PI. 5, Fig. 20. 



DISCOIDAL, subcompressed; sides flattened, converging towards 

 the dorsum, which is regularly rounded ; umbilicus about a third 



