December, 1843.] 333 



mesial furrow : cardinal area of the valves equal. Breadth, rather more than 

 half an inch. 



Locality. Ibid, (Trenton limestone.) 



Resembles the preceding, but differs in the equal hinge areas, in being much 

 more compressed, in having a proportionally longer hinge line, with more an- 

 gulated extremities, &c. 



Orthis. 



1. O. disparitis. Semi-circular, with about twenty-eight prominent round- 

 ed regular ribs ; larger ralve rather prominent in the middle ; the sloping sides 

 flattened ; middle rib bifid ; lesser valve slightly concave, somewhat depressed 

 or furrowed in the middle, the termination slightly contracting the base ; lateral 

 margins regularly rounded inwards from the angular extremities of the hinge; 

 cardinal area wide. Length, one-fourth of an inch ; breadth, one-third of an 

 inch. 



Locality. Ibid, (Trenton limestone.) 



2. 0. perveta. Transversely oval, wider than the length of the hinge line; 

 valves slightly ventricose, subequal, with numerous prominent radiating rounded 

 striae, bifurcated on the umbo ; larger valve ventricose in the middle, with a 

 slight central depression ; sides somewhat depressed ; the opposite valve flatten- 

 ed towards the base, and depressed to correspond with the elevation of the other 

 valve, forming a sinuous margin when viewed in profile ; base truncated ; su- 

 perior lateral margin obliquely truncated, rounded inferiorly. Length, one-third 

 of an inch ; breadth, nearly half an inch. 



Locality. Ibid, (Trenton limestone.) 



3. 0. tricenaria.. Semi-oval, with about thirty prominent very regular 

 rounded ribs; larger valve ventricose ; summit elevated; the dorsal margins 

 subrectilinear, very oblique; lesser valve flat or slightly concave in the middle ; 

 cardinal area very wide ; apex of the larger valve profoundly elevated above that 

 of the opposite valve. Length, three-fourths of an inch. 



Locality. Ibid, (Trenton limestone,) Lower Silurian. 



4. 0. bellarugosa. Semi-oval ; valves nearly equally convex ; lesser valve 

 with a mesial subangular furrow ; ribs prominent, linear, with unequal bifurca- 

 tions ; disks with numerous concentric prominent subsquamose wrinkles ; apex 

 of larger valve not much elevated above that of the opposite valve; cardinal area 

 rather wide. Length, less than half an inch. 



Locality. Ibid, (Trenton limestone.) 



5. O. subequata. Semi-oval ; valves ventricose, subequal ; lesser valve with 

 a slight subangulated mesial furrow ; larger valve prominent in the middle, with 

 flattened sides; radiating striae fine, closely arranged, unequal, rounded, cardinal 

 area rather wide ; apex of larger valve prominent, not profoundly elevated above 

 the opposite beak ; the dorsal margins concave. Length, half an inch. 



Locality. Ibid, (Trenton limestone.) 



EtJOMPHALUS. 

 E. triliratus. Discoidal ; volutions margined by a profound channel and a 

 carinated line and with a wide concave furrow from this line to the carinated 



