MERISTELLA OF THE UPPER HELDERBERG GROUP. 301 



Theinterior of the dorsal valve shows a strong cardinal process, with 

 a shallow spoon-shaped depression in the centre, margined by deep teeth 

 sockets. The muscular area is elongate-ovate, broader above, and divided 

 through the centre by a thin elevated septum. These features are shown 

 in fig. 17; and also in the casts of the dorsal side, figs. 1, 2, 20 and 22. 



The crura apparently bend downwards from near their origin ; 

 thence recurving, they follow very closely the contour of the dorsal 

 valve, making thirteen or more turns in an individual of medium size. 

 Spires, slender and simple. The spires and median septum are shown 

 in figure 19. 



In the young shell, this species is symmetrical and nearly circular; the 

 smallest measured specimen has a length and width of about half an inch. 

 The valves are very regularly convex, the dorsal valve becoming gibbous 

 in the upper part, with a scarcely perceptible undulation of the 

 front margin. Some specimens of an inch in length have an equal width; 

 but generally as the shell becomes older, the length exceeds the width. 

 The nasute extension in front is at first acute in most specimens, becom- 

 ing broader and linguiform in older specimens. 



The figures on Plate xlviii, from 7 to 16 inclusive, show the prevailing form and features 

 of the genus. 



The specimen illustrated in figures 23-25 is that which I have dcscnhei as M. elissa ; 

 but this is only an extreme form of M. nasuta with an unusual linguif trm extension, a 

 remarkably gibbous and arcuate ventral valve, and a dorsal valve less convex, as in some 

 smaller individuals. 



This species first appears in the Schoharie grit, where it is sometimes very fully 

 developed, but frequently is only obtained in meagre casts, like figures 3 and 4. 

 It is more abundant in the Corniferous limestone, here presenting its greatest 

 extremes in form and proportions. 



This shell is described by Mr. Billixgs, loc. cit., under the name of Atliyns 

 clara ; but the same has been long known as Atrypa nasuta of Conrad, and I 

 preserve that specific name as having priority. 



Both in general exterior form and internal characters this species bears much 

 resemblance to Meristella (^Atri/pa) lumida, Dalmax ; but in that one there is a 

 more distinct sinus iu the ventral valve, while the umbo is more gibbous, the 

 beak larger and more incurved, and the entire shell is comparatively more 

 ventricosc. The muscular area in the ventral valve is of the same shape, but 

 narrower than the prevailing form in our species. 



