ACROTRETID^. 665 



Waagen [1885, p. 750] describes the dorsal valve from specimens on a slab of sandy shale 

 that are more or less crushed. The surface of these shells is marked only by very fine concentric 

 strise and two show an opening beneath the apex similar to that of the ventral valve of Dis- 

 cinolepis granulata. I do not find in the material any clearly defined dorsal valve with the papil- 

 lose surface. The material representing the concentrically striated species is too poorly pre- 

 served to warrant the description of a second species. I do not know of any related species. 



I am indebted to the director of the Geological Survey of India, Dr. T. H. Holland, for the 

 opportunity to study and illustrate the type material of this genus and species. 



Formation and locality. — Middle Cambrian: Lower portion of the "Neoholus beds" of the Khussak group, in 

 purplish-colored, fine-grained, micaceous sandstone at the following localities [Waagen, 1885, p. 751]; (357a) at Jutdna; 

 and (357c) near the fresh-water springs in the gorge above the salt mines at Kiura (Khewra); both in the Salt Range, 

 India. 



Subfamily ACROTRBTIN^E Walcott and Scliucliert. 

 Genus LINNABSSONELLA Walcott.a 



Linnarssonella Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 601-602. (Described and discussed as below as a 



new genus.) 

 Linnarssonella Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 146. (Classification of 



genus.) 



Ventral valve convex with a slightly incurved beak projecting over a low false area. 

 Pedicle opening at the apex of the beak. A very slight trace of a pseudodeltidium occurs 

 beneath the beak, dividing the false area midway, as in the genus Micromitra; the false area 

 arches slightly upward and backward in some specimens, while in others of the same species 

 its edge is nearly coincident with the plane of the edge of the shell. The true cardinal area is 

 well defined and strongly marked by a shallow pedicle furrow (PI. LXXVIII, figs. 8e and 9b). 

 The dorsal valve is shghtly convex, with a minute beak at the posterior margin. Surface 

 marked by very fine concentric strise and undulations of growth. Shell strong, thick, and 

 built up of a thin outer layer and numerous inner layers or lamellae that are arranged more or 

 less obhquely to the outer layer. All the known species are small, not exceeding 2.5 mm. in 

 diameter. 



The cast of the interior of the ventral valve shows the presence of two well-marked car- 

 dinal scars, one on each side of the main vascular canals well toward the posterior border of 

 the valves. The main vascular canals of the ventral valve were large, extending nearly to the 

 frontal margin, and including between them back of the center of the valve a small visceral 

 area. The cast of the pedicle opening occurs just in front of the union of the main vascular 

 sinuses. The cast of the interior of the dorsal valve (PI. LXXIX, fig. 1 1) shows two large 

 cardinal scars; two central scars (li); and traces of a minute anterolateral scar (j); strong 

 vascular canals; a well-defined area and pedicle furrow; and a narrow median ridge extending 

 in some shells to the anterior third of the valve. 



Type. — Linnarssonella girtyi Walcott; second species, L. minuta (Hall and Whitfield); 

 third species, L. tennesseensis Walcott. 



This is a most interesting type, combining characters of Micromitra and Acrotreta. Bicia, 

 of the Olenellus fauna, has a strikingly similar dorsal valve, and the ventral valve is not unlike 

 if the narrow pedicle furrow of Bicia is closed so as to provide a pedicle aperture. 



Linnarssonella girtyi occurs in great abundance in a single layer in the Middle Cambrian 

 of the Black Hills, North Dakota; also at two localities at the same relative stratigraphic 

 horizon in Oklahoma and at several locahties in the Middle Cambrian limestones of Missouri. 



a The synonymy for this genus does not give a complete record of the various genera under which the species now included in Linnarssonella 

 were formerly placed; it gives only those references in which the genus is discussed or described. To complete the record the following mere generic 

 references are listed: 



Lingulepis? Hall and Whitfield [1877, p. 206). 

 Lingulepis? Walcott [1884b, p. 13). 

 Obolella? Walcott [1897a, p. 404]. 

 Lingulella Schuchert [1897, p. 257]. 



Obolella Walcott [1899, p. 443]. 

 Acrotreta Walcott [1901, p. 673]. 

 Meekina Walcott [1905a. p. 313], 



