OBSERVATIONS ON THE GENUS ATRYPA. 143 



distant toeth, is solid; the inner face being excavated, foiming a smooth, 

 depressed or concave area which extends about one-cightli of an inch below 

 the apex (Pl. I, fig. 2), the curvature conforming to that of the l)eak of 

 the opposite valve, which closely fills it. (A similar depressed area is 

 shown by Prof. Hall to exist in Rhynchonella incrcheace ils.) In young 

 and immature individuals, there uppeurs to have been a perforation passing 

 beneath this depressed area ; and perhaps in its earlier stages of growtU 

 the deltidial portion may have been of separate pieces, hut in adu ts it is 

 one solid thickened mass ; being, as Dalman supposed, actually imperfo- 

 rate. I do not remember to have seen this feature in iiny other variety or 

 species of the genus. Those of the uppei- rocks, of which separate valves 

 are not uncommon, are always extremely thin at this point; the deltoidal 

 space usually being open, from the loss of the deltidial plates at an early 

 period of g.owth. 



Tn the limestones of the Tapper Heldcrberg group, at the Falls of the 

 Ohio, there occurs a form which is much larger than those of the Lower 

 Helderberg limestones. The ventral valve of this v.iriety is flattened, and 

 usually a little concave toward tlie front, and the surface rather coarsely 

 striate, especially in the upper part of the shell. In this one, so far as 

 seen, the spiral ribbon is broad, the junction of the crura with it abrupt, 

 and at the posterior limit of the first volution ; the loop is direct-jd forward 

 and reaches upward to some distance, but not so far as in those from the 

 Shaly limestones. The volutions of the spires appear to be about fourteen 

 or fifteen (Pl. I, fig. 3). 



'J'he variety occurring so abundantly in the soft shales of the Hamilton 

 group of New York, is similar to the last in form and size, the otricC rather 

 finer. In a very perfect individual of moderate size, the loop is not so 

 much elevated in the middle as in that one ; the volutions of the spiral 

 cones number twenty-two, and are very closely arranged ; the attachment 

 of the loop and principal band, nearly the same (Pl. I, figs. 4 and 5). 

 There is another variety found in rocks of this age at Independence, Iowa, 

 which is often of large size and extremely ventricose on the dorsal side : 

 the striae are fine, and closely arranged. In a specimen of this variety, 

 the volutions are only fifteen, and the upward curvature of the loop 

 very slight. 



In Atrypa spinosa, Hall, the loop curves upward but little, and 

 extends forward more than in A. reticularis from the same beds, while the 

 volutions of the spir(,' are about fifteen. The principal distinction is in the 

 form of the spiral cones, and in the ribbon (Pl. I, figs. 6 and 7). Ouc of 

 the cones in the specimen used has been injured, which may have had some 

 influence in producing the modifications of the apex. The form of thj 

 cone diff"ers in having the upper part curved ; the apex being directed 

 backward, or towards the beak of the shell: the first basal volutions are 

 nearly straight on their inner sides, and the anterior extremity acutely 

 pointed, while the ribbon is abruptly widened near this part. 



I have been inclined to believe that the diff"erence in the loop and spires 

 mentioned above may prove to be of some value in determining whether 

 these difl"erent forms are really distinct species, or only changes superin- 

 duced by a diff"erence of conditions existing at certain localities during the 

 life of the animal. I have not yet carried the investigations far enough to 

 satisfy myself fully of the permanency of these internal difi'erences in 

 specimens of the same type. The fact of difl"crent varieties being often 

 restricted to a certain geological position is, however, good grounds lor 

 supposing that the peculiar internal features may also be retained. 



