ACROTRETIDiE. 709 



ACROTRETA SCHMALEKSEEI Walcott. 

 Plate LXX, figures 1, la-s, 



Not Acrotreta socialis von Seebach, 1865, Zeitschr, Deutsch. geol. Gesell. for 1865, Bd. 17, p. 341, PL Villa, figs. 



1^. (This species is referred in this monograph to Acrotreta socialis.) 

 Acrotreta socialis Linnarsson [not von Seebach], 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.) 

 ?Acrotreta socialis Linnarsson, 1877, Geol. Foren. i Stockholm ForhandL, No. 40, Bd. 3, No. 12, p. 374. (Not figured,, 



and may belong with either Acrotreta schnalenseei or A. socialis.) 

 Acrotreta socialis Linnarsson, Wallerius, 1895, tTndersokningar ofver Zonen med Agnostus Ixvigatiis i Vestergotland,. 



p. 66. (Discussed in Swedish.) 

 Acrotreta schmalensei Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 597-598. (Described as below as a new species.) 



As in the case of many other species of this genus, the student is referred to the series 

 of figures ilkistrating it for information as to the external outline, form, and convexity of the 

 two valves and the range of known variation. The collections of the United States National 

 Museum contain a large series of specimens from which the shells illustrated were selected. 



The broad false area has a shallow, faint median groove on some specimens, and on others 

 no traces of it have been seen. The pedicle aperture, which is exceedingly minute, is situated 

 at the extreme apex of the ventral valve; the cast of the base of the pedicle tube is clearly 

 shown in iirterior casts of the veJve (PI. LXX, figs. If and Ih). One of the distinguishing 

 characters, is the cast of the large main vascular sinuses on each side of the visceral area, a 

 feature which varies (PI. LXX, figs, le, li, and Is). The casts of the interior of the dorsal 

 valve (figs. Ij-lp) are interesting and instructive in showing the considerable variation in 

 the size and length of the median ridge, and the size of the cardinal and central scars. 



The shell is small and built up of a thin outer layer and several inner layers or lamella 

 more or less oblique to the outer layer. The outer surface is marked by fine concentric striae 

 and lines of growth, and the inner surfaces of the lamellse by concentric strife and fine radiating 

 striae. The average length of the opening of the valves is L5 to 2 mm., and the width is usually 

 a little more. 



Ohservations. — All of the specimens illustrated are from the Paradoxides forcJihammeri 

 zone. They appear to be identical with the figures of Acrotreta socialis given by Linnarsson 

 [1876, PI. Ill, figs. 32-35], but are not like those given by von Seebach [1865, PI. Villa, figs. 

 1-4]. Those of the latter represent a shell with strong concentric striae and median groove 

 on the false area, characters that Linnarsson states that he did not find on his specimens. 

 Among the collections made for me by Mr. G. Schmalensee, a collector in the Geological Sur- 

 vey of Sweden, I find a larger Acrotreta than the one described by Linnarsson [1876, p. 16], 

 which agrees with the description of A. socialis given by von Seebach [1865, p. 341] and with 

 his figures. Linnarsson writes that he thinks that von Seebach had representatives of several 

 species before him when he wrote his notes on A. socialis. With this I fully agree, but with 

 the collections now before me from Bornholm and Oeland, I think the specific name A. socialis 

 should be given to the larger shell, illustrated by von Seebach [1865, PI. Villa, figs. 1-4], and 

 a new name given to the smaller and very distinct shell illustrated by Linnarsson [1876, PI. 

 Ill, figs. 32-35]. In recognition of the faithful and intelligent work of Mr. G. Schmalensee, I 

 take pleasure in naming the species in his honor. 



Acrotreta scJimalenseei is of the type of A. suhconica Kutorga, but is much less elevated. 

 It may be compared with A. microscopica (Shumard) and A. gemma Billings among American 

 species. 



Formation and locality. — Middle Cambrian: (323b) Limestone of the Paradoxides celandiciis zone, at Wind- 

 juelandet, Ringsaker, Province of Hedemerken, Norway. 



(320n) Limestones of the Paradoxides forchhammeri zone at Lovened, Djupadal, 19 miles (30.6 km.) south-southeast 

 of Skara; (320e) limestone .'it Munkesten, north of Hunneberg; (331q) limestones of the Paradoxides forchhammeri zone 

 at Munkesten, north of Hunneberg; and (320m [Linnarsson, 1876, p. 18]) limestone of the Paradoxides forchhammeri 

 zone at Kinnekulle, northeast of Lidkoping; all in the Province of Skaraborg, Sweden. 



(8w) Limestones of the Paradoxides forchhavivieri zone at Andrarum; and (320c [Wallerius, 1895, p. 66]) Andrarum 

 limestone, at Andrarum; both 20 miles (32 km.) northwest of Simrishamn, Province of Christianstad, Sweden. 



