OBSERVATIONS ON THE GENUS ATRYPA. Igg 



connecting the spires, more than two years ago. I had hoped to continue 

 them much farther than I have yet done, previous to making known the 

 results ; but circumstances have made it necessary to publish them at 

 the present time. 



The differences described below, are some of the principal ones noticed 

 in the several varieties under which they are given. Of their importance, 

 I leave others to judge. 



The loop was first noticed in immature specimens from the Niagara 

 group, occurring at Waldron, Indiana. In the adult specimens from this 

 locality, the usual form of the shell is lenticular, with the dorsal valve a 

 little the most convex; the surface not very finely ribbed, but very 

 squamose. The ribbon, or band forming the spiral cones, is wide; the 

 junction of the loop with the principal ribbon is at a point distant from 

 their attachment to the hinge-plate, and not far from the point of greatest 

 width of the shell. The loop is strong and but moderately curved 

 upward, with a broad gentle curvature. The spire, in a specimen of 

 moderate size, consists of about twelve volutions; the cones are erect, 

 their apices a little posterior to the centre of their bases. 



In the Shaly limestones of the Lower Helderberg group, we have a 

 form with a very ventricose dorsal valve, the ventral being only mode- 

 rately so : the surface is rather more finely ribbed than in the preceding 

 variety, but not so squamose. In this one the ribbon is slender, and the 

 junction of the loop much nearer to the hinge-plate ; while the loop is 

 extended into a long slender point, slightly recurved near its extremity, 

 and reaches to about half the height of tlie spiral cones. The cones con- 

 sist of about fourteen volutions, and are situated very much as are those 

 of the Waldron specimens (Plate I, fig. 1). 



Besides the difference of the external characters of this variety, as 

 well as of the loop and spires, there is a very noticeable peculiarity in 

 the beak of the ventral valve. The entire beak, to the outer limits of the 

 widely distant teeth, is solid ; the inner face being excavated, forming a 

 smooth, depressed or concave area which extends about one-eighth of an 

 inch below the apex (Plate I, fig. 2), the curvature conforming to that 

 of the beak of the opposite valve, which closely fills it. (A similar 

 depressed area is shown by Prof Hall to exist in Rhynchonella increbescens.) 

 In young and immature individuals, there appears to have been a perfo- 

 ration passing beneath this depressed area ; and perhaps in its earlier 

 stages of growth the deltidial portion may have been of separate pieces, 

 but in adults it is one solid thickened mass ; being, as Dalman supposed, 

 Cab. Nat. 22 



