Silurian Fossils. 21 



Stropheodonta muxdula, sp. nov. 

 Plate II. Fig. 18. 



Brachial valve concave, pedicel valve convex. Width 

 about 1-i mm., length 9 mm., depth approximately 2 to 2.5 

 mm. Surface covered with very fine radiating stricT, visible 

 under a lens^ of which some at more or less regular intervals 

 are more conspicuous. Of the latter there are about 9 in a 

 width of 4 mm. near the anterior margin. 



Cardinal process not preserved. Immediately in front of 

 the position which should be occupied by this process there 

 is a thickened area 1.3 mm. in width which narrows anteriorly 

 to a median ridge about a third of a millimeter in width, and 

 which thickens again slightly toward the end. This ridge 

 may be traced to a point 6 mm. from the beak ; slightly over 

 7 mm. from the beak the valve, as seen from the interior, Is 

 deflected downward and this marginal part preserves the 

 traces of vascular markings. From the broad, thickened area 

 near the beak one very divergent low ridge extends off on 

 each side at an angle of about 20 degrees with the hingeline, 

 limiting the posterior part of the posterior diductor impres- 

 sions. These are separated from the anterior diductor im- 

 pressions by indistinct ridges which extend from the same 

 broad, thickened area near the beak forward at an angle of 

 about 25 degress with the median line. The anterior part 

 of the anterior diductors may be traced to a point about 3 mm. 

 from the hingeline. while the posterior diductors extend to a 

 point about 2 mm. from this line. Corresponding to the indis- 

 tinct ridge between the anterior and posterior diductors, an 

 irregular ridge extends forward and becomes thickened and 

 more conspicuous anteriorly. It may be traced, on each side 

 of the valve to a point 5 mm. from the beak. The thickened 

 terminations of the two divergent ridges just described, and 

 that of the median ridge form characteristic features of this 

 species. The space between these terminations and as far 

 back as the hinge line is distinctly granulose or papillose, as in 

 the interior of Strophoinena vetusta. Indistinct traces of 



21 



