38 NEW YORK STATE MUSEUM 



are so interrupted by the radial ribs on the body of the shell as to form 

 radial rows of deep meshes which often alternate in their position in adjoin- 

 ing rows. The meshes in C . i n c a r c e r a t u m are much the larger trans- 

 versely. The anterior ridge is sharply elevated and crested, the anterior 

 slope very abrupt, excavated and striated by the elevated concentric lamellae 

 which here take on a radial attitude. The posterior termination is extended 

 and acute and the valves gape at this end. These specimens show very 

 clearly the structure of the sculpture or prismatic layer of the shell in these 

 species, which is rendered distinctly cavernous by the projection of the con- 

 centric growth in the form of pronounced lamellae rising from the deep 

 intervals between the ribs and dividing these areas into series of elongate 

 pit-shaped meshes. 



I have been disposed to regard these shells identical with C. rhen- 

 a n u m as described and figured by Beushausen \pp. cit. p-4O2, pi. 30, fig. 

 5-8]. There is agreement between the two in respect to size, form and 

 radial markings but the lamellar surface structure is not defined in sufficient 

 detail to determine whether it corresponds to that of the Dalhousie shell or 

 that of C . i n c e p t u m Hall. As there is a palpable difference herein we 

 have preferred to give these shells a distinctive designation. Conocar- 

 dium rhenanum is from the Coblentz quartzite and the Upper 

 Coblentzian of the Rhine. 



Horizon. No. 1 1. 



Other pelecypod remains occur in these strata, some of them indicating 

 notable species but the specimens before us are not in favorable preservation. 



Rensselaeria stewarti Clarke 



Plate 7, figures 10-20 



Rensselaeria stewarti Clarke. N. Y. State Mus. Bui. 107. 1907. p. 239 



Shell naviculate, the unequal convexity of the valves being very 

 marked. The ventral valve is highly convex and arched, the line of greatest 

 curvature being median from which the slope is somewhat abrupt to the 

 sides giving the valve a subcarinate exterior. The umbo of this valve 

 is high and overarched, projecting far beyond the hinge line, the apex 

 being incurved and truncate. The cardinal area is represented by a flat- 

 tened triangular area free of striae and rather definitely delimited. The 

 dorsal valve is gently and evenly convex with low and inconspicuous umbo 

 and beak. The surface of both valves is covered by abundant subequal 

 radial riblets all of which are simple and continuous from beak to margin 

 except in rare instances where additions are introduced. There is consider- 

 able difference in the coarseness of the radial marking in mature shells, the 

 number being as low as 40 and as high as 80 to 90 on each valve. 



The radial lines are crossed by exceedingly fine concentric striae. On 



