Caprina Limestone Beds. 



107 



cross-striae, which are the exterior terminals of the circum- 

 scribant septa? of the honeycombed 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 by the intersection of the concen- 

 tric and vertical laminae. The imbricate concentric lamina? are 

 arranged in successive layers diverging upward from the interior 

 layer of the shell. The interior shell (6) is thin and very poorly 

 preserved, being largely replaced by calcite crystals. 



FIG. 1. Longitudinal section of larger extremity of specimen figured on 



plate .riii. 



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

 same; c, a cross-septum of interior cavity; d, last chamber of interior 

 cavity ; e/ e", calcified area, marking position of the muscular apophysis 

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

 of apophysis ; g. undetermined fragment. 



The opening of the shell is composed of the thin interior 

 shell and a few layers of the exterior shell. Most of the con- 

 centric lamina) of the latter gradually disappear before reaching 

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

 half centimeters deep, and the 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 upper valve. 



