DIELASMA 257 



exhibit all the essential generic characters of Dielasma most perfectly. 

 The interpretation of the genus by Hall and Clarke 1 is identical with 

 that here given, but those authors included certain species in the genus 

 without sufficient investigation of their internal characters, which are 

 really fundamentally different ; it has in fact been the usual custom 

 among American workers, since the publication of Hall and Clarke's 

 work, to refer all Mississippian terebratuloid shells to the genus Dielasma. 

 In specimens preserved in the condition of internal casts the generic 

 characters of Dielasma are always very obvious, the position of crural 

 lamella?, separate from the socket plates being indicated by a pair of 

 slits diverging from the beak of the brachial valve ; when the transverse 

 muscle-bearing plate is attached along its mesial line a second pair of 

 diverging slits are present between those formed by the crural lamellae 

 and the finger-like casts of the slender cavities beneath the transverse 

 plate are clearly shown, whether they are actually present or broken off. 

 In specimens having the shell preserved the shell substance is frequently 

 translucent enough to show the position of the internal lamella? as dark 

 lines, in which case the genus can be recognized at once, and when the 

 shell is opaque it is usually easy to determine the generic characters by 

 the judicious use of a needle, without injuring the specimen as to its 

 external form and characters upon which the various species are dif- 

 ferentiated. 



DIELASMA CHOUTEAUENSIS n. sp. 

 Plate XXXII, Figs. 1-17 



3895. Dielasma for m ,osa Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 81, 

 fig. 24 (not figs 12-23, 25-26). 



Description. Shell of medium size or larger, subovate or obscurely 

 subpentagonal in outline, the greatest width near the middle, the anterio'r 

 margin rounded. The dimensions of a nearly complete example are: 

 length of pedicle valve 32 mm., length of brachial valve 29.3 mm., greatest 

 width 23.2 mm., thickness 15 mm. 



Pedicle valve most convex posterior to the middle, arched from beak 

 to front along the median line with the curvature becoming progressively 

 greater posteriorly, the surface convexly curved from the median line 

 towards the postero-lateral margins for a little more than half way to the 

 margins where it is rather abruptly inflected to a slight degree, along a 

 narrowly rounded, arched ridge which originates at the side of the beak 

 and terminates at the lateral margin posterior to the middle of the valve, 

 beyond this arched ridge the inflected surface is usually gently concave, 

 to the lateral, antero-lateral and anterior margins the surface is gently 

 convex; the mesial portion of the valve is usually slightly flattened be- 



1 Pal. N. Y., vol. 8, pt. 2, pp. 293-294. (1894.) 

 9 



