200 NORTH AMERICAN INDEX FOSSILS. 



XII. Acrothele Linnarsson. 



Corneous, subcircular. Pedicle valve subcortical with excentric 

 beak truncated by the pedicle opening, anterior to which are often 

 two wart-like protuberances. Two such protuberances also form 

 the umbo of the brachial valve which is marginal. Differs from 

 Acrotreta in having the slope from the apex to the posterior margin 

 quite gentle, and in the absence of a cardinal area. Cambric. 



35. A. matthewi (Hartt). (Fig. 234, /-#■.) Middle Cambric. 

 Round, extremely flat and thin. The two small wart-like pro- 

 tuberances are present in front of the foramen of the pedicle 

 valve. 



St. John beds of Newfoundland, New Brunswick. 



36. A. subsidua (White). (Fig. 234, d-e.) Lower to Middle Cambric. 

 Thin, with transverse diameter slightly exceeding the longitud- 

 inal. Sides regularly and front broadly rounded. Posterior mar- 

 gin forms a short and nearly straight hinge line. Brachial valve 

 flat; pedicle valve somewhat convex near the umbo, with rather 

 prominent beak. 



Utah, Nevada. 



37. A. gamagei (Hobbs). Middle Cambric. 

 Differs from A. matthewi in its more nearly circular form, longer 



hinge-line and in having the surface marked by radiating striae. 

 Surface flat or slightly convex instead of concave in anterior 

 portion. 



Middle Cambric Paradoxides beds of eastern Massachusetts. 



XIII. LlNNARSSONIA Walcott. 



Calcareous, subcircular, convex. Beak of pedicle valve excen- 

 tric, perforated by a minute pedicle opening. No cardinal area 

 on either valve. Differs from Obolella in the absence of the great 

 lateral muscular impressions and cardinal areas, and also in the 

 presence of a pedicle opening in the pedicle valve. It is smaller 

 and more conical than Acrothele. Cambric. 



38. L. pretiosa (Billings). Upper Cambric. 

 Breadth exceeding length with greatest width about the middle. 



Beaks very obtuse. Surface covered with very fine lamellose con- 

 centric striae. Length about ^ inch. 

 Quebec. 



