MISSISSIPPIAN LOOP-BEARING BRACHIOPODA 441 



its anterior margin which are usually discontinuous along the 

 median line. 



Remarks. — The genus Dielasma was established by King with 

 Terebratula elongatus Schl. as genotype, and although he defined 

 the genus primarily upon the presence of prominent dental lamellae 

 in the pedicle valve, and on the form of the loop, his illustrations 

 of the internal casts of the species under the name Epithyris elongata 1 

 show that the crural plates are separate from the socket walls, one 

 of the most essential features of Dielasma as here defined. David- 

 son 2 gives illustrations of the same species which exhibit all the 

 essential generic characters of Dielasma most perfectly. The 

 interpretation of the genus by Hall and Clarke 3 is identical with 

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

 genus without sufficient investigation of their internal characters, 

 which are 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 posi- 

 tion of the crural lamellae, 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 speci- 

 mens having the shell preserved the shell substance is frequently 

 translucent enough to show the position of the internal lamellae 

 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 injur- 

 ing the specimen as to its external form and characters upon which 

 the various species are differentiated. 



1 Monog. Perm. Foss. England, PI. 6, Figs. 37, 41 (1850). 



2 Brit. Foss Brack., II, Permian, PL 1, Figs. 18, 20. 

 sPal. N.Y., VIII, Pt. 2, pp. 293-94. 



