Cnprina Limestone Beds. 



107 



cross-stria^ which are the exterior terminals of the circum- 

 scribant septa^ of the honeycomhed structure. The shell is 

 composed of two layers (see fig. 1). The outer one (a) is very 

 thick and constitutes most of the substance and has a fine cellu- 

 lar honeycombed structure. Cells very minute and rectangular 

 in cross-section and produced 1»y the intersection of the concen- 

 tric and vertical lamina^. The imbricate concentric laminiie are 

 arranged in successive layers diverging upward from the interior 

 layer of the shell. The interior shell (h) is thin and very poorly 

 preserved, l^eing largely replaced 1)\' calcite crystals. 



Fig. 1. — LoDgitudinal ftcrfion of large i' exfrenillg of specimen figured on 



plate .riii. 



a, outer shell of larger valve ; h, space once occupied by inner shell of 

 same; e, a cross-septum of interior cavity; (7, last chamber of interior 

 cavity ; e^ e" , calcified area, marking position of tlie muscular a^iophysis 

 of upper valve ; /, small fragment of upper valve ; J' , section of outer edge 

 of apopliysis ; 7, undetermined fragment. 



The opening of the shell is com})osed of the thin intcri(n- 

 shell and a few layers of the exterior shell. j\Iost of the con- 

 centric laminte of the latter gradually disappear before reaching 

 this termination. The dwelling chamber is about one and one- 

 half centimeters deep, and tlie details of its structure somewhat 

 concealed by the filled-in matrix. A longitudinal section of the 

 large extremity (fig. 1) gives no detail of the anatomy of the 

 living chamber, but gives some light on the uj)per valve. 



