DENTALIUM-FISSIDENTALIUM. 67 



D. MEGATHYRis Dall. PI. 15, figs. 29, 30, 31. 



Shell remarkably stout and solid, rapidly enlarging ; the earlier 

 third moderately curved, the remainder much straighter. Surface 

 where not eroded shining ; texture of shell porcellanous within, with 

 an external chalky stratum under the smooth exterior ; the poste- 

 rior half generally much eroded even in living specimens. Color 

 yellowish-white, generally with some dark extraneous matter lodged 

 in the interstices. Sculpture: numerous (about 50) strong longitudi- 

 nal riblets and threads, the latter rather sparsely and irregularly 

 interposed ; the intervals deep and generally somewhat narrower 

 than the riblets ; longitudinals rather abruptly losing in strength 

 near the aperture in aged shells. Apertui-e (fig. 29) decidedly ob- 

 liqiie, somewhat wider than long, the peristome subsinuous, acute. 

 Apex with simple, circular, sharp-edged orifice (fig. 31), No slit or 

 notch. 



Length 99, greatest diara. of aperture 18'1 mill. 



Length 97, greatest diam. of aperture 17'9, diam. at apex 2*7 mill. 



Length 95, greatest diam. of aperture 17*5, antero-posterior diam. 

 15-5 mill. 



Length 91, greatest diam. of aperture 18, antero-posterior diam. 

 16-5 mill. 



Off Chiloe Island and southeast Chili in 1050 and 1342 fras. 

 near Galapagos Is. in 812 fras. ; off Ecuador in 1740 fms. ; Gulf of 

 Panama ; s.-iv. of Tehauntepec, 2282 fms. ; off Mazatlan, 995 fms. ; 

 Gulf of California off La Paz, (U. S. Fish Comniission). 



Dentalitim megathyris Dall, Proc. U. S. Nat. Mus., xii, p. 293, pi. 

 9, f. 1.— Stearns, Proc. U. S. Nat. Mus., xvi, p. 424 (1893). 



This is one of the finest species of the genus, and the stoutest 

 known. Dall writes : " The young recalls D. ceras Watson, but the 

 shell changes in rate of increase and form of longitudinal ribs as it 

 grows. It is a little straighter near the anal end, and the adult is 

 more funnel-shaped, with flatter ribs than in D. ceras. 



"The radula is short, with the formula . . ^ . The median 

 tooth is wide, subrectangular, arched a little in front. The laterals 

 on each side have a projecting stout cusp ; the uncini are flat rhom- 

 boidal plates. The whole radula bears a strong resemblance to that 

 of Entalis striolata as figured by G. O. Sars. The oesophagus is 

 short; the stomach short and cordate, stuffed with foraminifera. 

 The soft parts, as preserved in alcohol, seem ridiculously small and 

 out of proportion to the massive shell." 



