256 MISSISSIPPIAN BRACHIOPODA 



Genus DI EL ASM A King 



Description. Shell terebratuliform. Pedicle valve with or without a 

 median sinus, and with well-developed dental lamellas internally, the 

 foramen large and encroaching upon the umbonal portion of the valve, 

 the beak strongly incurved. Brachial valve usually without mesial fold ; 

 internally the crural plates are separate from the dental socket plates, 

 they diverge from the apex of the valve with an elongate attachment to 

 the inner surface of the valve, the free portion of the brachidium is 

 short with diverging descending lamellag ; between the crural plates for 

 the full length of their attachment to the inner surface of the valve, is 

 a concave, transverse plate for muscular attachment, which joins the 

 inner surface of the crural plates a little above their bases, this plate rests 

 against the inner surface of the valve along the median line for a portion 

 or the whole of its length, or may be free throughout, when attached 



V V V V 



N 



FIG. 28. A series of fourteen cross-sections of the rostral portion of the brachial 

 valve of Dielasma formosum (X 2%), showing the development of the crural 

 lamellae entirely independent from the hinge-plate, and the transverse, muscle- 

 bearing plate between them. 



along the median line a pair of slender cavities, triangular in cross section, 

 converge from the general cavity of the shell towards the beak ; when the 

 transverse plate is not attached along its median line there is a single, 

 broad and low cavity beneath the plate extending towards the apex, 

 anteriorly this plate extends to a greater or less distance beyond the 

 attachment of the crural plates and is pointed, rounded or emarginate in 

 front, its surface is marked by concentric wrinkles parallel with its an- 

 terior margin which are usually discontinuous along the median line. 



Remarks. The genus Dielasma was established by King with Terebratula 

 dongatus 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. Davidson 2 gives illustrations of the same species which 



IMon. Permian Foss. Eng., pi. 6, figs. 37, 41. (1850.) 



2 Brit. Foss. Brach., vol. 2, Permian, pi. 1, figs. 18, 20. (1857.) 



