DIELASMA 265 



than half way between the median line of the valve and the lateral mar- 

 gins, the crural plates are joined transversely by a concave plate bearing 

 the muscle scars, which is in contact with the inner surface of the valve 

 medially, leaving a pair of finger-like, triangular cavities converging and 

 diminishing in size towards the beak, each bounded by the base of a crural 

 plate, the muscle plate and the inner surface of the valve. 



Surface of both valves smooth posterior to the line of geniculation, but 

 anterior to this line it is marked by more or less conspicuous concentric 

 lines of growth. The shell structure finely punctate. 



Remarks. This species is one of those which has com'monly been referred 

 to Girtyella turgida, but besides possessing characters of the rostral portion 

 of the brachial valve which are generically different from that species, the 

 external configuration of the shell is very different. The shell attains a 

 much larger size than G. turgida, and differs from it in the geniculate habit 

 of growth of the shell, and also in the much more conspicuous triplication 

 of the brachial valve towards the anterior margin. The more conspicu- 

 ous variations observed in the species are the proportional width of the 

 shell and the depth of the mesial sinus. In some examples the width and 

 thickness of the shell is nearly equal, while in others the thickness ex- 

 ceeds the width to a notable degree. The internal characters of the 

 rostral portion of the brachial valve have been determined only from the 

 markings of the septa as seen upon the exfoliated external surface, but 

 these clearly indicate the genus Dielasma. 



Horizon. Salem limestone. 



DIELASMA FORMOSUM (Hall) 

 Plate XXXIII, Figs. 12-17 



1856. Terebratula formosa Hall, Trans. Albany Inst., vol. 4, p. 6. 

 1882. Terebratula formosa White, llth Ann. Rep. Geol. Surv. Ind., p. 361, 

 pi. 39, figs. 6-8. 



1882. Terebratula formosa Whitfield, Bull. Am. Mus. Nat. Hist., vol. 1. 



p. 55, pi. 6, figs. 59-64. 



1883. Terebratula formosa Hall, 12th Ann. Rep. Geol. Surv. Ind., p. 337, 



pi. 29, figs. 59-64. 



1894. Dielasma formosa Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 53, figs. 15-17 (not 18-19). 



1895. Dielasma formosa Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 81, 



figs. 18-23, 25-26 (not figs. 12-17, 24). 

 1895. Dielasma cbovata Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 81, 



figs. 38-40. 

 1897. Dielasma obovata Hall and Clarke, 14th Ann. Rep. N. Y. State Geol., 



p. 372, pi. 14, figs. 12-14. 

 1906. Dielasma formosum Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1310, 



pi. 22, figs. 59-64. 

 1911. Dielasma formosa Weller, Jour. Geol., vol. 19, p. 440, figs. la-n. 



