712 CAMBEIAN BKACHIOPODA. 



Not Atrotreta sodalk Linnarsson, 1876, Bihang till K. svensk. Vet.-Akad. Handl., Bd. 3, No. 12, pp. 16-18, PI. Ill, 



figs. 32-35. (Described and discussed in English. This species is referred in this monograph to Acroireta 



schmalenseei.) 

 fAcrotreta socialis Linnarsson, 1877, Geol. Foren. i Stockholm Forhandl., No. 40, Bd. 3, No. 12, p. 374. (Not figured, 



and may belong with either Acrotreta socialis or Aerotreta sdmialenseei.) 

 Acrotreta socialis von Seebach, Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 599-600. (Described and discussed 



essentially as below.) 

 Not Acrotreta cf. socialis Matthew, 1902, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 5, No. 20, pp. 392-394, PI. 



XV, figs. 5a-k. (Described and discussed. This species is referred in this monograph to Acrotreta.ep. undt.) 

 Not Acrotreta cf. socialis Matthew, 1903, Geol. Survey Canada, Rept. Cambrian Rocks Cape Breton, pp. 183-185, 



PI. Ill, figs. 5a-k. (Copy of preceding reference. This species is referred in this monograph to Acrotreta sp. 



undt.) 



The species described and illustrated by von Seebach [1865, p. 341, PI. Villa, figs. 1-4] as 

 Acrotreta socialis is characterized by a well-marked, concentrically striated surface and a strong 

 median groove in the false area. In the collections made for me by Mr. Schmalensee at Borg- 

 holm, Oeland Island, I find associated with Paradoxides mlandicus an Acrotreta that has these 

 characters. All the specimens from the Paradoxides forcliliammeri zone in the large collection 

 before me, with the exception of one ventral valve, belong to a distinct species which I have 

 named A. schmalenseei. Von Seebach evidently had specimens of the latter species, also of a 

 species of Acrotreta from the Ceratopyge limestone, as he mentions the occurrence of A. socialis 

 at both horizons. 



The external form, as far as known to me, is shown by the figures. There is some variation 

 in the height of the ventral valve and in the outline of the margins of the valves. The false 

 area is clearly defined and marked by a strong, rather broad median groove. The pedicle 

 aperture is at the apex of the ventral valve and quite readily seen in several specimens, a char- 

 acter in strong contrast with the minute aperture at the apex of A. schmalenseei Walcott. 

 Shell formed of several thin layers or lamellae that show very plainly where the thin outer laj^-er 

 is exfoliated. Surface marked by striae and lines of growth that are very distinct on some 

 shells and less so on others; the concentric striae occur on all the lamellae of the shell and on 

 the inner surface; fine radiating striae are to be found on the inner surface. The cast of the 

 interior of the ventral valve shows the presence of a large apical callosity and unusually large 

 pedicle tube and main vascular sinuses on each side of the visceral cavity. The cardinal scars 

 are well defined in both the ventral and dorsal valves. Casts of the interior of the dorsal valve 

 show a strong median ridge, central scars, and fairly well-defined main vascular sinuses. Von 

 Seebach [1865, p. 341] describes the surface as having minute warts on it. . I find numerous 

 fragments of the shell of Acrothele (Redlicliella) granulata (Linnarsson) associated with Acrotreta 

 socialis, and it may be that von Seebach mistook the surface of this shell for that of the species 

 he was describing. No known species of Acrotreta has such a surface. 



A large ventral valve has a diameter of 5 mm. and a height of 2.5 mm. The average size 

 is about 3 mm. in diameter. 



A single ventral valve found by Schmalensee at Andrarum appears to belong to this species. 

 It is a cast of the interior of the shell, and shows concentric ridges or undulations of growth, 

 well-defined cardinal scars, large apical callosity, and short but strong main vascular sinuses 

 (PL LXXIII, fig. 4b). 



This species belongs to the A. subconica Kutorga group of species with a broad false area 

 and well-defined median groove. Its surface is more strongly marked by concentric stiise than 

 any other species of the genus, and the shell is also thicker. 



Gronwall [1902, p. 39] and Linnarsson [1877, p. 374] refer specimens to Acrotreta socialis, 

 but do not accompany the references by descriptions or figures, and I therefore can not decide 

 whether to include the forms with Acrotreta socialis or Acrotreta sclimalenseei. 



Matthew [1902b, p. 392] has compared some imperfect specimens of a species of Acrotreta 

 from McNeil Brook, Cape Breton, with Acrotreta socialis von Seebach. The shells, as far as 

 can be determined, do not belong to this species. They are about the same size, but the ventral 

 valve is depressed and more like that of Acrotreta sagittalis magna (Matthew) . I can not identify 



