FOSSILS OF THE CLINTON GROUP. 593 



the flattening of the posterolateral slides is sufficiently extended to give 

 the shell a more triangular outline; this appearance is increased by the 

 fact that these sides usually form much less than a right angle at the beak, 

 at times only 70°. Corresponding to this difference, the shell is less strongly 

 convex, the sinus and fold are narrower, all the plications are narrower 

 and closer, a fact which can be readily remarked even in the young 

 when only 7 mm. long. The height of the fold and depth of the sinus 

 is usually less, these median regions not unfrequently projecting a little 

 beyond the anterior outline of the shell. In a fourth form the outline is 

 quite rotund, but the shell is still less convex, so as to show in 

 comparison with the ordinary types, a considerable flattening, even in 

 adult shells, so that a shell 16 mm. long may show a depth of only 7 mm, 

 this flattening being still more marked in smaller specimens. Rarely 

 these flattened specimens have lower and broader plications, giving still 

 another aspect to the shell. The surface of all these forms is ornamented 

 by very fine concentric strise uf growth visible under a tense. These 

 striae of growth are interrupted at regular intervals. When these intervals 

 are longer, the striae are formed by a succession of short, fine, minute ridges; 

 when shorter^ the striae are reduced to a succession of minute granules; the 

 latter are rather more common, though both forms may be seen on the 

 same shell. The intervals of interruption occur with such regularity as 

 to produce that quincuncial arrangement of granules already remarked 

 while describing the' similar surface ornamentation of Orthts biforata 

 The species is found in considerable abundance at Huffman's Quarry _ 

 Soldiers' Home, Centerville, more rarely at Fauver's, Todd's Fork, and 

 rarely at Brown's Quarry, in the quarry in John Glaser's woods five and 

 a third miles northeast of Dayton, on Brandt pike. 



Rhychonella acinus, var. convexa, Foerste. 



( Plate SI, Fig. 13, a, 6, c.) 



Found at Todd's Fork, Soldiers' Home and more commonly at 

 Hanover, Indiana, in the Clinton. Described in the Proc. Bost. Soc. 

 Nat. Hist, for May 1, 1889. 



Cyclospira ? sparsi-plica a, sp. ncv. 



(Plate 37A, Figs. 18, », b.) 



At Huffman's Quarry a shell was found with the pedicle valve exceed- 

 ing the brachial in convexity and depth, the beak of the former being 

 also much more incurved and, its umbo more elevated. The posterior 

 half of the valves is evenly convex. The anterior half is plicated, es- 

 pecially towards the anterior margin of the shell, the general effect being 

 the production of a sort of broad median sinus in the brachial valve and 

 a corresponding elevation in the opposite valve. Two broad plications 



38 G. O. 



