FOSSILS OF THE CLINTON GROUP. 591 



indicate a much broader form, the general outline of the pedicle valv 

 being more ovate and much less elongate, the sides being not at all com 

 pressed. In other respects this form agrees exteriorly with the last 

 there being no antero-lateral angulation, no tendency of the anterioi 

 border to be strongly sinuate, nor to flatten out so as to suggest a tri- 

 angular outline for the shell. The surface is marked by fine concentric 

 striae of growth. At Hanover, Indiana, a small entire shell was found 

 which seems to belong to this form. In outline these shells most nearly 

 resemble Meristella Prinstana, Billings. From Meristella intermedia they 

 differ in the more rounded outline of the postero-lateral parts of the shell. 



Atrypa marginalis, Dalman. 



(Plate 25. Fig-. ; Fig 9. Plate 31. Figs S 9,9 a.) 



This species, though quite variable, presents certain general char- 

 acteristics which are readily recognized and which are quite character- 

 istic. Thus the median fold of the brachial valve which is fairly 

 high and strong anteriorly, begins as a simple plication near the beak, 

 and may retain its simple character for almost four or five millimeters 

 before it begins to divide. Moreover the fold is defiued laterally by a 

 bordering line of depression, these lines of depression or grooves being 

 most distinct posteriorly, where they not uncommonly meet so that in 

 those cases the median fold starts as a simple narrow plication a slight 

 distance anterior to the beak. This median ridge usually does not begin 

 to rise above the general surface of the shell, at least to any marked de- 

 gree, until it is 5 to 7 mm. distant from the beak, so that for a time it has 

 rather the appearance of resting in a sort of median sinus. The pedicle 

 valve begins to develop a median sinus close to the beak, which becomes 

 deep anteriorly. In forms from Brown's Quarry, the sinus remains 

 narrow for quite a distance, increasing but moderately in depth, after 

 which the sides curve outwards and the depth of the sinus increases 

 rapidly. This valve shows a tendency towards flattening laterally. At 

 Huffman's Quarry, Fauver's and at Soldiers' Home a slightly different 

 form occurs in which the sides of the sinus diverge more regularly, and 

 the sinus therefore increases in depth earlier. The pedicle valve is 

 usually a little more convex. 



Variety multi- striata from Hanover, Indiana, (Plate 31, Fig. 8.) has 

 still more divergent sides to the sinus, and the number of radiating pli- 

 cations is somewhat greater. Aside from the radiating plications the 

 surface of the valves of this species shows only more or less strong con- 

 centric wrinkles. 



Atrypa lati-corrugata, sp. no v. 



'Plate 37A,'Figs, 16, a, b, c, d.) 



In the "Orthoceras block" found at Huffman's Quarry, shells were 

 found which on superficial examination might be taken for a form of 

 Atrypa retiadaris, but which a closer view shows to be intimately related 



