yo Aug. F. Focrste 



interior of the shell of Clitambonitcs divcrsns is covered by coarse 

 iiiiCrtwining vascular markings, which in CHtainbonites rogerscnsis 

 are comparatively inconspicuous. The shell of CHtainbonites 

 rogersensis is broader and shorter. The pedicel valve, in conse- 

 quence, appears less convex, and the beak appears less conspicuously 

 elevated. The brachial valve is transversely elongate, while that 

 of Clitanibonites divcrsns is subquadrate in outline. The anterior 

 outline of Clitanibonites rogersensis not infrequently is slightly re- 

 entrant or concave, but this is not a constant feature. 



Opisthoptera concordensis, sp. nov. 



(Plate I, fig. 9.) 



Along the creek east of Concord, Kentucky, the Strophoniena 

 concordensis zone is almost at the base of the actually exposed sec- 

 tion, a short distance south of the railroad bridge. Five and a half 

 feet lower, the lowest specimens of Stveptclasma vagans were 

 found, associated with a species of Opisthoptera. Five feet higher, 

 Streptelasma vagans and Columnaria alveolata occur. Seven and 

 a half feet above the Strophomena concordensis layer, the species 

 of Opisthoptera occurs again, associated with the lowest specimens 

 of Dalmanelld jugosa found at this locality. A specimen of 

 Opisthoptera collected from the higher horizon just mentioned pos- 

 sesses the following characteristics : 



Greatest length of the left valve, from the beak to the ventral 

 border, 75 mm. Greatest convexity of this valve, 15 mm. Byssal 

 opening well defined. About 20 primary plications reach the beak. 

 Along the more elevated part of the valve, within 40 mm. from the 

 beak, these primary plications are dichotomously divided into two. 

 Anteriorly and posteriorly this division takes place nearer the beak. 

 Along the ventral third of the main body of the shell, an additional 

 plication occupies the depression between the pairs of more promi- 

 nent plications resulting from this dichtomous division. Posterior- 

 ly, the intercalated plications have about the same prominence as 

 the pairs resulting from the division; in consequence, the shell ap- 

 pears here more closely and evenly plicated. Anteriorly, the pos- 

 terior division of the primary plication appears more prominent. 



This species appears closely related to Opisthoptera oUernata, 

 Ulrich. It is distinguished by the paired appearance of the dicho- 

 tomously divided primary plications over the larger part of the 

 body of the shell. 



