ABT.l. RUDISTID SHELLS FROM MEXICO — STEPHENSON. 6 



shell layer had entirely disappeared before fossilization, so that no 

 trace remains of the hinge mechanism, adductor scars, or other inter- 

 nal features ; with the inner shell layer gone the interior of the lower 

 valve, after the removal of the matrix, is a nearly circular cavity 

 which gradually decreases in diameter from above downward. 



The upper rim of the shell into which the uper valve fits is steeply 

 inclined, forming a flaring aperture ; the surface of the rim is slightly 

 undulating and bears a few faint impressions of radial vessels, some 

 of which are branching; these are spaced 5 to 20 mm. ajDart; this 

 surface is also covered with numerous very fine, faint, radiating 

 striae, and portions of the surface are finely pitted ; the inner margin 

 of the rim is bordered by a slightly raised sharp ridge which is 

 closely paralleled by a narrow channel. 



The outer surface of the shell bears two siphonal channels (E and 

 S, pi. 1), which vary somewhat in character on different individuals 

 in the two species described in this paper. Where the incremental 

 lines cross these channels they bend downward toward the base of 

 the shell. 



The anterior siphonal channel is shallow and is marked along the 

 bottom throughout its length by the trace of a narrow sinus which, 

 in the form of a closely pressed slit, cuts back through the shell 

 at right angles to the surface, extending to within 3 or 4 mm. of the 

 inner surface (pi. 4, a) ; this is a line of weakness along which the 

 shell easily breaks and splits apart (see pi. 8, a) ; the sinus is some- 

 times a little open, admitting a thin seam of matrix. Where this 

 sinus intercepts the upper rim of the shell it forms a distinct slit, 

 generally slightly gaping, which is bordered by a narrow carina 

 (pl.2,«). 



Both in front and behind the sinus the surface of the rim exhibits 

 broad, shallow depressions, each of which is roughly a right-angle 

 triangle with its base resting on the margin of the inner cavity of 

 the shell. The two triangular areas are connected with each other 

 by a flat uncarinated space between the inner end of the sinus and the 

 inner margin of the shell (pi. 2). A polished cross section (pi. 4) 

 shows that all the growth layers bend sharply inward as they ap- 

 proach this sinus. The hypotenuse of each of the triangular areas 

 is also marked in the polished cross section by the disposition of the 

 growth lines which along the trace of the triangle bend obliquely 

 outward, due to the thinning or entire disappearance of the inter- 

 mediate cells for a distance of 1 to several millimeters (pi. 4). This 

 modification in the growth lines doubtless resulted from the con- 

 tractile movements of the ventral siphon. 



The upper rim of the shell between the posterior siphonal channel 

 and the margin of the inner cavity is also marked by a broad, shal- 



