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TEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 17, 



is a trifid shelly process (fig. 2, b), noticed above as giving attach- 

 ment to the adductor muscles ; but as this lies above the middle 

 portion of the outer loop (c), while it occupies the hollow (fig. 3, b) 



Figs. 1-3, illustrating Thecidium Adamsi. 

 Fig. 1. 



Fig. 1 . Superior view of the exterior. 



a. Ventral valve. 



b. Dorsal valve. 



c. Umbone. 



• d. Hinge-area 



e. Deltitium. 



Fig. 2. Inferior view, with a por- 

 tion of the ventral valve 

 (a) broken away to show- 

 the adductor process {b), the 

 outer plain loop (c), the 

 intermediate half- loops (d), 

 and the median crest (/). 



Fig. 3. Interior view of the dorsal 

 valve. 



a. Cardinal process. 



b. Recess for the adductor 

 process of the ventral valve. 



c. Hinge-tooth. 



d. Granulated margin. 



e. Outer simple loop. 



/. Intermediate half-loops. 

 g. Inner reflected loop. 

 h. Median crest. 

 i. Points of origin of the parts 



composing the internal 



framework. 



in front of the cardinal process (a) of the dorsal valve, it follows that 

 when the shells are intact they cannot be forcibly separated without 

 breaking the loop (fig. 2, c) upon the adductor process (b). I there- 

 fore, very cautiously, broke away the ventral valve from below, as 

 shown in fig. 2, and by simply studying the relations of the parts, I 

 was enabled to perfect the scheme of the loop-apparatus, as given in 

 fig. 3. 



The hinge-area (fig. 1, d) slopes downwards and a little back- 

 wards from the line of union of the valves to the beak; and 



