THE LOWER SILURIAN ROCKS. 663 



pressure being about as three to eleven. The thickness is proportionally 

 a little less, being about 7 mm. in a specimen 12 mm. high. More im- 

 portant difference is found in the mesial sulcus and umbonal ridge, both 

 of these features being less distinct than in C. typicalis. Because of the 

 very slight development of the mesial sulcus, the ventral margin, except 

 at the ends, is perfectly straight, and thus furnishes us with another 

 diagnostic point since in that species the base is gently concave. The 

 umbones again are smaller and not so distinct from the cardinal region 

 on each side of them. Finally the ends of the shell are somewhat dif- 

 ferent, the anterior one being more uniformly rounded and the posterior, 

 straighter above causing a sharper post-cardinal angle. 



Cymatonota pholadis, or at any rate a form that we identify with 

 Conrad's Orthonota pholadis, is closely related and associated in the same 

 beds with C. recta. It is however a more elongate shell, the length be- 

 ing quite five times the height, while its valves are more convex, giving 

 the entire shell a sub-cylindrical appearance that is quite foreign to 

 C. recta. 



The Orthonata (later Orthodesma) parallella of Hall (Pal. N. Y. 

 Vol. I, p. 299; 1847), includes two or three distinct forms. They are 

 badly illustrated and insufficiently described, and it is scarcely safe to 

 draw conclusions respecting their generic affirmities without access to 

 the original specimens. While at least one probably belongs to Cyma- 

 tonota, I am satisfied that all of them are widely different from the 

 species here described. 



Formation and Locality. — Middle beds of the Cincinnati group, at 

 several localities in the immediate vicinity of Cincinnati, Ohio. The 

 species has a vertical range of about 60 feet. 



Cymatonota s<mistriata, n. sp. 



Plate 55. Figs. 6 and 7. „ 



Length about 35 mm., greatest height about 11 mm. Posterior end 

 subtruncate, base very gently convex, anterior margin sloping backward 

 almost from the right angled cardinal extremity. Umbonal ridge rather 

 distinct and subangular in the upper half; mesial sulcus not developed. 

 Anterior half of surface marked with very fine, thread-like, regular con- 

 centric lines. The majority of these lines cease suddenly before reach- 

 ing the umbonal ridge, Oblique cardinal furrows smaller and more 

 numerous than usual. Three faint lines diverging from the beak may 

 be noticed on the cardinal slope between the umbonal ridge and dorsal 



border. 



This beautiful shell is readily distinguished from C. typicalis and C. 

 recta by the shape of the anterior end and the more abrupt ceasing and 



