578 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



1884. AmAreta Walcott, Moiig. U. S. Geol. Sur., VIII, p. 16. 

 1886. Acrotreta Matthew, Trans. Roy. Soc. Canada, III, p. 36. 

 1886. Acrotreta Walcott, Bull. U. S. Geol. Sur., No. 30, p. 98. 



1891. Acrotreta Walcott, Tenth Ann. Rept. U. S. Geol. Sur., p. 608. 



1892. Acrotreta Hall and Clarke, Pal. New York, VIII, Pt. 1, p. 101; Eleventh 

 Ann. Rept. State Geologist, p. 250. 



1885. L'mnarssonia Walcott, Amer. Jour. Sci., 3d ser., XXIX, p. 115. 



1886. Lmiiurssonia Matthew, Trans. Roy. Soc. Canada, III, p. 35. 

 1889. Linnarssonia Dawson, Trans. Roy. Soc. Canada, VII, p. 55, fig. 26. 



1892. Linnarssonia Hall and Clarke, Pal. New York, VIII, Pt. 1, 1892, p. 107; 



Eleventh Ann. Rept. N. Y. State Geologist, 1891, p. 251. 

 1892. Linnarssonia Matthew, Trans. Roy. Soc. Canada, II, p. 42. 

 1902. Acrotreta Matthew, Bull. Nat. Hist. Soc. New Brunswick, IV, Pt. 5, pp. 



390, 394. 



Original description. — Dorsal valve highly conical; the hinge surface of this cone 

 flat, in the form of a high triangle, similar to an area, with a shallow gutter-shai^ed 

 depression running from the tip as far as the middle point, which [depression] here 

 appears as an indication of a deltidium. At the upper end of this furrow, turned 

 consequently to the hinge side, is found the obtuselv oval external siphonal open- 

 ing(I). 



Ventral valve flat, with a distinct marginate apex. On the surface of the shell are 

 seen only delicate growth wrinkles concentric to the apex of the cone, which curves 

 crescentically into the longitudinal furrow of the surface of the shell; no tubercles 

 and no spines; hinge border rectilinear. 



Revised generic description. — Ventral (pedicle) valve conical, with 

 the posterior face more or less flattened, and usually marked by a 

 shallow groove. Foraminal opening- at the apex of the cone and 

 directed slightly backward. Area narrow, divided midway liy the 

 path of advance of a small false deltidium. Dorsal (brachial) valve 

 slightly convex, with very small beak; area short and divided as in the 

 pedicle valve by a small false deltidium'. Surface marked In' line 

 concentric striae and lines of growth which cross the posterior face 

 and the median groove. The shell in all species where it is preserved 

 is calcareo-corneous, and built up of several thin laj'ers of lamella 

 that are arranged more or less obliquely to the outer surface toward 

 the outer margin of the valves. 



The cast of the pedicle valve shows that the interior of the shell had 

 a rather strong callosity or apical swelling penetrated by the foraminal 

 tube, and on each side of and back of the callosity near the posterior 

 margin a small projecting boss or cardinal tubercle, which corresponds 

 to a depression in the shell, on which the transmedian and middle 

 lateral muscle were probabl}- attached. In front of the apical cal- 

 losit}' in A. argenta there are two trapezoidal areas corresponding to 

 similar areas in OboleUa and Oholus, in which the central, outside, and 

 middle lateral muscles were attached. The position and size of the 

 areas are shown by several specimens. The grooves of the main 

 vascular sinuses pass around the apical swelling and extend forward, 

 diverging toward the antero-lateral margins of the shell. The interior 



