128 REV. J. E. CEOSS ON THE GEOLOGY OE N.W. LINCOLNSHIRE. 



horizon, viz. the upper portion of the zone of A. Bucklandi, where it 

 is associated with A. semicostatus and the Tancredia described above 

 (T. ferrea). 



HippopoDruM perm (Cross, MS.), Ether. PI. V. figs. 6, 6 a. 



Shell oblong, thick and ponderous, obtusely truncated anteriorly, 

 rounded posteriorly, dorsal edge arched or slightly convex, ventral 

 margin sinuated ; umbones large, thick and incurved, hinge-line 

 thickened, especially near the umbo ; lines of growth following the 

 sinuosities of the ventral margin. The characteristic oblique tooth is 

 not clearly observable in our specimens, owing to the matrix cover- 

 ing it up ; neither are the muscular scars or pallial line. 



Affinities and Differences. — This shell differs essentially from the 

 type IT. ponderosum, being much more acute at the posterior end, 

 less ponderous, and less rugose. 



Log. Scunthorpe. Formation. Lower Lias (Ironstone series). 



Cucfll^ea Rolandi (Cross, MS.), Ether. PI. Y. fig. 3. 



These small Cucullcece are at all times difficult to name, age 

 having much to do with specific determination ; but there is that 

 about the above-named form which prevents us from confusing or 

 identifying it with any known species. 



Shell small, rhomboidal or quadrate, equilateral and convex; 

 umbones central, moderately large, nearly touching anterior side ; 

 broadly rounded ; posterior side with an obtuse carina and flattened 

 space ; ventral margin convex and smooth ; hinge-line straight, 

 narrower than shell ; area lanceolate, marked by 5 divaricating stria3 

 or lines ; lateral hinge-teeth four or five on either side of umbo, 

 horizontal ; none on the median or central portion, which is thin and 

 well defined. Surface of shell faintly marked by lines or plications 

 of growth. 



This shell differs from C. santonensis in the structure and arrange- 

 ment of the dental system, in not having any reticulation upon the 

 posterior side near the umbo, and in the absence of central teeth 

 along the hinge-area between the lateral teeth. 



Affinities. — The rounded equilateral and obtuse form and umbo of 

 this shell resembles that of Limopsis ; but the horizontal and less 

 numerous teeth remove it from that genus ; with no known Cucullcea 

 can I correlate it. A shell named 0. fabiformis by Terquem and 

 Jourdy, from the Great Oolite of Les Classes (Dept. Moselle), resembles 

 our C. Rolandi in some respects ; but the hinge-line is straighter 

 and longer, and the shell more quadrate. 



Log. Santon. Formation. Inferior Oolite. 



Cucull^a santonensis (Cross, MS.), Ether. PL Y. f. 2, 2 a. 



Shell obtuse or quadrate, inequilateral ; umbones nearly central, 

 contiguous ; area beneath umbones narrow and traversed by three 

 divaricating obtuse-angled ridges ; lateral teeth 5, strongly defined 

 and slightly inclined ; central or median teeth 3, and small. Posterior 

 side truncated; ventral margin smooth and thin; hinge-area or line 

 much narrower than the width of the shell along ventral border, 



