248 ORGANIC REMAINS. 



into three or four slender ribs. The same, or a similar shell, occurs 

 in the upper shale. Another very large shell is met with in the oolite 

 and the grey limestone, resembling a. ingens, but having the aperture 

 more square, and the ribs more acute, each of vrhich terminates 

 towards the back in a sharp knob. This shell, of which we have no 

 entire specimen, may be termed a. aciiticostatus. 



No. 12, PI. XIV, is a much smaller shell, from the blue limestone, 

 or rather from alluvial nodules of blueish limestone, having also very 

 sharp ribs, but continued round the back ; and having the aperture 

 round. The original shell is brown with some white spots, as in «. 

 Hildensis; so that we may name this species a. maculatus. A similar 

 species, or a variety of the same species, also from the alluvium, has 

 on the sides two small knobs upon each rib ; the one towards the 

 back, and the other, which is the smallest, towards the inside. 



No. 13, PI. XIV, has also sharp ribs, and has a sharp keel run- 

 ning along the back. It occurs in the lowest shale at Robin Hood's 

 Bay, and other places. A large shell generally found imperfect, but 

 apparently of the same species, occurs in the Redcar rocks. The 

 a. Sticklandi of Sowerby, Tab. 130, seems to be a cast of this species. 

 We would prefer naming it a. Redcarensis. 



No. 6, PI. XIII, is a very small but handsome ammonite, also 

 from the lowest shale, and common in Robin Hood's Bay. Its ribs 

 are sharp on the sides, but thick and flat on the back. As it is 

 generally found in a pyritous state, shining like gold, we might call it 

 «. aureus; but it seems to be the same with a. planicosta of Sowerby, 

 Tab. 73. 



We have met with no shell corresponding exactly with Mr. 

 Sowerby's a. armatus, Tab. 95 ; which he states to exist in our alum 

 shale, in numerous varieties. We have several distinct species of 

 knobbed ammonites, but have seen none with striated knobs, stuck 

 on like bai-nacles or small patellae, as in the outer whirl of Mr. 



