OBOLID^. • 477 



This form owes its specific name to its association with Agnostus. 



Limestone in passage beds between 

 yienus truncatus zone; at Borgholm, c 



LiNGXJLELLA AMPLA (OwCn). 



Formation and locality.— (310i) Limestone in passage beds between the Middle Cambrian Paradoxides forch- 

 Jiammeri zone and the Upper Cambrian Olenus truncatus zone; at Borgholm, on Oeland Island, Sweden. 



Plate XXVIII, figures 1, la-j. 



lAngula ampla Owen, 1852, Eept. Geol. Survey Wisconsin, Iowa, and Minnesota, p. 583, PI. I b, figs. 5 and 12. 

 (Described as a new species, see p. 478 for copy. The specimen represented by fig. 5 is redrawn in this mono- 

 graph, PL XXVIII, fig. 1.) 



Lingula ampla Owen, Hall, 1863, Sixteenth Kept. New York State Cab. Nat. Hist., p. 125, PL VI, fig. 10. (Original 

 description, Owen, 1852, p. 583, copied and species discussed.) 



Lingula ampla Owen, Hall, 1867, Trans. Albany Inst., vol. 5, pp. 101-102, PL I, fig. 10. (Discussion and figure copied 

 from preceding reference.) 



Lingulella ampla (Owen), Schuchert, 1897, Bull. U. S. Geol. Survey No. 87, p. 257. (Merely changes generic reference.) 



Obolm (Lingulella) amplus (Owen), Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 392 and 394, PL XXVIII, 

 figs. 3-4. (Mentioned in the text in discussion of Lingulella. The specimens represented by figs. 3 and 4 are 

 redrawn in this monograph, PL XXVIII, figs. If and Ih, respectively.) 



General form ovate, with the ventral valve obtusely acuminate and dorsal valve broadly 

 ovate; valves moderately convex. Surface of shell marked by concentric strise, and lines of 

 growth and indistinct radiating sti'ise ; very fine radiating striae occur on the inner surface of 

 the outer layer of the shell, and the casts show the presence of papillse that indicate that the 

 inner surface of the shell was punctate (PI. XXVIII, fig. Ig). The shell is of medium thickness 

 and formed of a thin outer layer and several inner laj^ers or lamellse which give a greater thick- 

 ness to the posterior half. The lamellse are arranged as in Lingulella acutangula (Roemer) 

 where they are slightly oblique to the outer layer. 



One of the larger ventral valves has a length of 18 nun., with a maximum width of 12 mm., 

 and an associated dorsal valve has a width of 12 mm. and a length of 15 mm. 



As shown in the cast of the interior of the shell, the area of the ventral valve is nearly flat 

 near its lateral margin and rises from about the flexure line toward the pedicle groove. The 

 pedicle furrow is shown by a narrow, elevated, rounded, tapering ridge which divides the area 

 midway. The area is marked also by rather strong flexure lines which extend from the beak 

 obliquely across the area at some distance from the outer margin, as shown in Plate XXVIII, 

 figures Ic-f. The striae of growth cross the area parallel with its base; they are very fine and 

 closely groiiped together on the lateral slopes; none of the specimens show their presence on 

 the cast of the pedicle furrow. The area forms a thin shelf between the pedicle groove and 

 the lateral margins, the undercut extending backward far imder the area. This is well shown 

 in several of the casts, although the cast of the imdercut is usually broken away. A portion 

 of it remains, however, in Plate XXVIII, figure Ic. 



The area of the dorsal valve is well defined and is marked in one specimen by strong flexure 

 lines (f, fig. Ih), and fine transverse strise. As in the ventral valve, the area forms a thin shelf, 

 the cast of the undercut extending well under the area in several of the specimens. Its ragged 

 edges are shown in figure li. 



The cast of the interior of the ventral valve shows the visceral cavity (v), and there are 

 slight traces of a median septum (s) in the dorsal valve (PI. XXVIII, fig. Ih). The muscle 

 scars are partly preserved. In the ventral valve the anterior laterals (j) are shown in one cast 

 (PI. XXA^II, fig. Ic). The central, middle lateral, and outside lateral muscle scars are all 

 crowded together and lost in the space in front of the anterior portion of the visceral cavity. 

 The central (h) and anterior lateral (j) muscle scars are well defined in the dorsal valve (PI. 

 XXVIII, figs. Ig and Ih). 



Of tjie mairkings left on the cast by the vascular system only the main sinus (vs) is pre- 

 served (PI. XXVIII, figs. Ic, Id, and If for the ventral valve and figs. Ig and Ih for the dorsal 

 valve). 



Observations. — One of the specimens illustrated by Owen [1852, PI. I b, fig. 5] is in this 

 monograph represented by Plate XXVIII, figure 1 . It appears to be a ventral valve that 



