N0.1299. CAMBRIAN BRACHIOPODA—WALCOTT. 599 



3 aj of Brog-ger, at Kristiauia, Yestfonus, Eugervik, and doubtfully 

 at Vakkero near Kristiania, all in collection of Univer.sity of Kristi- 

 ania, collected by Dr. W. C. Brog'g'er, Norway. 



ACROTRETA SIGNALIS, new species. 



Pedicle valve transverse, broadly ovate; height about one-third the 

 length; apex directed backward on a line with the posterior margin, 

 which is transverse and slightly undulated at the center by the median 

 furrow of the slighth' defined false area; apex with a minute apical 

 foramen directed backward. Surface marked by fine concenti'ic strife 

 and lines of growth that at the median furrow of the false area curve 

 toward the posterior margin. Shell small, 1.5 mm. longf by about 1.75 

 mm. in width. Shell is built up of several very thin layers or lamelloe. 



In all of the great collections made from the Cambrian sandstones 

 of the Upper Mississippi Valley, onh-^ two pedicle vaives of an}^ 

 species of Acrotreta have been found, as far as known to me. These 

 occur in association with Oholus {Lingidejns) acuviinatus^ and the 

 shells have the color and appearance of those of that species. Shinv, 

 light gray to white, the original coloring matter having been leached 

 out. Onh^ the exterior of the pedicle valve is known. This compared 

 with A. microscojyica shows a less elevated pedicle valve and less dis- 

 tinct false area. It also has a marked median groove on the false area. 

 It recalls A. idahoensis, but difi'ers in being more depressed and in 

 having a stronger g-roove on a narrower false area. 



For mat ion and locality. — Middle Cambrian, St. Croix sandstone, 

 St. Croix Falls, Wisconsin. 



ACROTRETA SOCIALIS v. Seebach. 



Arrotrela socialis v. Seebach, Zeit8ch. deutschen geol. Gesellsch., XVII, 1865, 

 p. 341, pi. VIII, figs. 1-4. 



The species described and illustrated by Psofessor v. Seebach as Acro- 

 treta socialis is characterized b}' a well-marked concentrically striated 

 surface and a strong median groove in the false area. In the collec- 

 tions made for me by Mr. Schmalensee at Oland, Borgholm, I find 

 associated with Paradoxides olandicus an Acrotreta that has these 

 characters. All the specimens from the Paradoxides forchhammeri 

 zone belong to a distinct species which I have named A. schmalensei. 

 Professor v. Seebach evidently had specimens of the latter species, 

 also of a species of Acrotreta from the Ceratopyge limestone, as he men- 

 tions the occurrence of A. socialis at both horizons. As now restricted 

 ^1. socialis occurs at Oland in the Paradoxides olandicus zone. 



The external form, as fai- as known to me, is shown by the figures. 

 There is some variation in the height of the pedicle valve and in the 

 outline of the margins of the valves. The false area is clearly defined 

 and marked ])y a strong, rather broad median groove. The foramina! 



