120 AMERICAN JOURNAL 



externally, while the interior was nearly white ; while in others 

 the whole substance of the shell was evenly suffused with color, 

 inside and out. 



The incomplete foramen appears to be a constant character, 

 though the degree of approximation of the deltidia varies a good 

 deal. The area is almost always strongly marked and nearly 

 smooth. The lines of growth are usually strong and often im- 

 bricated, especially when the radiating ribs are strong. 



The hinge line is normally nearly straight, and recalls jSpirifer, 

 but in some adult and many immature specimens is very much 

 arched. The sinuosity of the margin caused by the broad neu- 

 ral mesial ridge is generally present, but varies very much in 

 degree, and is sometimes almost evanescent. When the ribs 

 are strong the margins are crenulated, and vice versa. The 

 number of ribs in an adult specimen is from thirty to forty. 

 They vary very much in prominence. Some specimens have 

 none at all ; others have half the shell smooth and half ribbed. 

 Some are ribbed only on the umbones ■, others only near the 

 margin. One specimen, which taken alone would certainly have 

 been considered distinct, is smooth, pink, with a prominent 

 apex, and short, bifurcating, varicose, impressed lines scattered 

 over the surface, a sharp carina on the neural and a correspond- 

 ing groove on the htemal valve. 



The shell is always thick, solid and strong, e\o.n in the smallest 

 specimens. The form presents every possible variation from 

 longitudinally oval to quadrispherically transverse. Many are 

 unsymmetrical, and most of them are overgrown with polyzoa^ 

 serpulae, and corallines. The peduncle is very short and stout. 



The apophyses are extremely delicate and fragile, and un- 

 usually small in proportion to the shell. The reflected portion 

 of the loop is very short, and the whole of the skeleton is broader 

 in proportion than in any other species known to me. The sep- 

 tum is very small and triangular, it diminishes on each side from 

 the point of attachment of the loop processes and is evanescent 

 in the umbo of the hjBmal valve. The most elevated point is 

 midway between the cardinal process and the anterior margin. 

 The cardinal process is inconspicuous. An indication of an ob- 

 scure mesial ridge may sometimes be seen in the neural valve. 

 The interior of the shell is sometimes a little perlaceous, but it is 

 usually without lustre. The punctures are smaller, more circular 

 and fewer in number than in T. pulvinata. An examination of 

 the young shells, only about -2 in. in extreme width, showed some 

 interesting points.* The septum is calcified very early and is 



*From the study of these immature forms I have come to the conclu- 

 sion that the genus founded by Prof. King on Tcrehratula capsida, Jetf., 



