PSEUDOSYRINX 409 



slope with from 12 to 14 simple, depressed, radiating plications which 

 originate along the cardinal margin and grow successively smaller towards 

 the cardinal extremities. Internally the dental lamella? are well devel- 

 oped and extend anteriorly from the beak along the floor of the valve for 

 one-half or nearly one-half the length of the valve to the anterior margin 

 on either side of the sinus, their divergence from the beak is a little 

 greater than the divergence of the lateral margins of the median sinus ; 

 the muscular scars are usually faintly impressed and are either not divided 

 longitudinally by a median ridge or by a very slight one; the delthyrial 

 plate is rather short, reaching one-third or less than one-third of the 

 distance from the apex of the delthyrium to the hinge-line, it is 

 nearly flat or slightly convex transversely with a regularly concave free 

 margin. 



Brachial valve shallower than the pedicle, the greatest convexity on 

 the median line at or near the front margin, the surface curving more 

 abruptly to the cardinal margin, somewhat compressed towards the car- 

 dinal extremities ; the mesial fold moderately elevated, rounded on top, 

 well defined to the beak; the beak small, projecting a little beyond the 

 cardinal margin, a little incurved ; cardinal area narrow and inconspicu- 

 ous, lying in nearly a right angle to the area of the opposite valve. In- 

 ternally a distinct median septum extends a short distance forward from 

 the beak and is then continued to about the middle of the valve as a slight 

 median ridge ; muscular scars obscure. 



The minute surface markings of the shell are not well shown upon any 

 of the specimens studied because of their condition of preservation, but 

 suggestions of the "twilled cloth" form of ornamentation is recognizable 

 upon some examples in patches; the shell structure is minutely punctate, 

 the perforations extending also across the cardinal area of the pedicle 

 valve to the border of the central, triangular, differentiated region where 

 they stop abruptly ; concentric lines of growth of varying strength are 

 usually present at irregular intervals upon both valves. 



Remarks. This species is the smallest member of the genus recognized 

 in our faunas, although it grows to a larger size than is indicated by the 

 dimensions given, (the largest individual which has been referred to the 

 species having a width of 46 mm. This largest specimen is an internal 

 cast of a pedicle valve in chert, and differs somewhat from the smaller 

 examples in having longer dental lamellae and in having the lingual ex- 

 tension of the sinus of greater length, but both these characters may be 

 due to the greater age of the specimen. The holotype of the species is a 

 nearly perfect internal cast from a white chert, its locality is not given 

 but it might come from many exposures of such chert in the Keokuk 

 formation of the Mississippi valley. The punctate structure of the shell 

 is finely shown upon it as upon most other similar chert specimens, by the 



