FOSSILS OP THE HUDSON EIVEE GEOUP. 89 



Genus SEDGWICKIA, McCoy. 

 Sedgwickia? divaeicata (n, sp.). 



Plate 2, fig. 3. 



Shell small, transversely elongate ovate, widest posterior to the mid- 

 dle; length of the shell about twice the width; beaks pointed, promin- 

 ent, and projecting above the hinge line ; flattened on the back and 

 sharply angular on the ridge ; valves somewhat compressed; hinge line 

 slightly arcuate, and extending to within a short distance of the greatest 

 length of the shell ; posterior extremity broadly and somewhat regularly 

 rounded ; basal margin gently and evenly convex to within a short dis- 

 tance of the anterior end, where it is more abruptly curved upward to 

 tho anterior extremity, which latter is a little above the center of the 

 shell and sharply angular, the upper side sloping rapidly from between 

 the beaks to the point of greatest extension. Valves distinctly angular 

 along the umbonal ridge, which extends in nearly a straight line from 

 the beak to the postero-basal margin ; above the ridge the cardinal slope 

 is wide, gently convex in the higher portions, but becoming compressed 

 toward the postero-cardinal region. Anterior to and below the umbonal 

 ridge the general surface of the valve is flattened, or very depressed 

 convex, with a gradual slope from the crest of the ridge to the border of 

 the shell. 



The surface of the valves is marked by two sets of ridges, the first 

 set forming concentric undulations parallel to the lines of growth, and 

 are, for the size of the shell, proportionally strong; posterior to and 

 above the umbonal ridge they are not so distinctly marked. The second 

 set exist in the form of strong, rounded plications, diverging from the 

 line of the umbonal ridge toward the base of the shell below, and to the 

 cardinal margin above, each of them curving very gently toward the an- 

 terior end in their course to the border of the valve. These plications 

 are mostly simple, but a few of them appear to bifurcate. 



This feature of diverging plications marking the surface of Lamelli- 

 branchiate shells of the Silurian formation is an exceedingly rare one ; 

 and it is somewhat remarkable that two species should occur in the same 

 formation, and in the same vicinity, having this form of surface orna- 

 mentation, Modiolopsis pholadiformis, Hall, being the other species. They 

 are, so far as we are aware, the only American forms from rocks of this 

 age having this style of marking. 



