FOSSILS OF THE CLINTON GROUP. 559 



point to which the shell can be traced is too near the plane of the junc- 

 tion of the valves to permit the idea that a concave line of surface inter- 

 vened between it and the upper margin of the shell. All the evidence is 

 therefore in favor of a rounded anterior outline, becoming concave above, 

 anterior to the beak, as in the figure 2>a, of plate 59, in the second volume 

 •of the Paleontology of New York, repesenting Aviczda subpla?ia. The 

 greatest length of the valve was 43 mm., the greatest height 36.5 mm., 

 the greatest depth of the valve 9 mm. 



In the collection of Mr. Geo. Caswell is a valve, which is evidently 

 that of an Avicula, which has a slight exterior resemblance to that of 

 Avicula Whitfieldi, but there is a stronger indication of an anterior wing, 

 and posteriorly along the hinge line a part of the exterior surface is still 

 preserved showing the presence of numerous fine, sharp, radiating, and 

 concentric striae. Its specific identity could not be determined. Plate 37, 

 fig. 6. 



Mytilarca mytiKformis, sp. nov. 



(Plate 37, Figs 11, a, b, c.) 



In the absence of all interiors to show the characters of the hinge 

 line or the teeth, the position of this shell can only be approximately 

 determined by means of its exterior surface. The shell is mytiloid, 

 and very gibbous. The beak is terminal. The anterior margin of the 

 valve extending almost the entire length of the shell is gently and quite 

 evenly convex. The hinge line p isterior to the beak is not long. The pos- 

 terior margin almost rounds into the hinge line; at first it is only moder- 

 ately convex but posteriorly it rounds more strongly, merging into the 

 anterior margin with a very convex curve. An umbonal ridge extends 

 with slight curvature from the beak to the posterior extremity. From 

 the beak it rises up strongly for a distance equalling one third the length 

 of the shell, after which it looses in elevation and distinctness. A 

 flattened, but gently rounded, surface is enclosed by this umbonal ridge 

 and the anterior margin of the shell. It is strongly inclined, making an 

 angle of about 60 degrees with the median plane between the valves. 

 Posteriorly the shell is quite regularly and not very strongly convex, ex- 

 cept towards the hinge line at which the curvature becomes concave where 

 the umbonal regions slope down towards the hinge line. The greatest 

 convexity of the shell lies therefore along the umbonal ridge, especially 

 towards the beak. Length of the valve here described 34 mm ; greatest 

 width 20 mm ; distance from the anterior end of the beak to the posterior 

 end of the hinge line, 14 mm; the greatest measurement in line of thick- 

 ness of the valve is the elevation of the umbonal ridge at one third the 

 distance from the beak above the margins of the valve: 12 mm. The 

 anterior margin makes a general angle of about 60 degrees with the 

 hinge line. The surface is marked by numerous but not very fine con- 

 centric striae. 



