680 



CAMBRIAN BEACHIOPODA. 



The original description by Moberg and Segerberg follows : 



The species differs from Acrotreta drcularis, to which in other respects it is closely related, by its more even area, 

 sharply bounded on the sides. At the rectilinear posterior edge the area occupies about one-fourth of the cu'cumfer- 



ence; that is to say, more than in the case of ^1. drcularis. The apex is 

 also placed somewhat nearer the posterior edge than in the last-named 

 species. The smaller shell is only feebly arched, with marginal, some- 

 what overlapping umbo. 



Bj' its large well-marked keeled area our species resembles A. 

 sodalis, from the Ceratopyge limestone of Norway. 



Observations. — This species has about the same size 

 and general appearance as Acrotreta seebacTii. It differs 

 from it in the more anterior position of the apex and the 

 strongly defined false area with a median ridge. 



Formation and locality. — Passage beds between the Cambrian 

 and the Ordovician: (310) Ceratopyge limestone (zone 4), atOttenby, on 

 Oeland Island; and (309) Ceratopyge limestone (zone 4), at Fogelsdtig, S 

 miles (5 Jem.) east of Lund, Province of Malmohus; both [Moberg and 

 Segerberg, 1906, p. 66] in Sweden. 



FiGUEE X.— Acrotreta carinata Moberg and Segerberg. 



A, Top view of ventral valve. B, Back view of 



ventral valve. C, Side view of ventral valve. D, 



Dorsal valve. All X 8. The ventral is the type. 



The specimens represented are from LocaUty 309, 



the Ceratopyge limestone at FogelsAng, Province of 



Malmohus, Sweden. The figures are copied from 



Moberg and Segerberg [1906, Fl. Ill, flg.s. oa-c and 0]. 



u- 



AcROTRETA ciRCULARis Moberg and Segerberg. 



Text figures 57A-C. 



On both sides of the 



Acrotreta drcularis Moberg and Segerberg, 1906, Medd. fran Lunds 

 Geol. Faltklubb, Ser. B, No. 2 (Aftryck ur K. Fysiografiska Sall- 

 skapets Handl., N. F., Bd. 17), pp. 65-66, PL III, figs. 4a-c. (De- 

 scribed in Swedish as a new species; see below for translation. 

 The figures are reproduced in this monograph as figs. 57A-C.) 



The original description by Moberg and Segerberg follows: 



The apex of the larger shell (ventral valve) is situated midway between the posterior edge and the center, or possi- 

 bly (the apex itself is not preserved) somewhat closer to the center. On the posterior edge there is a slightly defined 

 triangular area which has a median rounded ridge extending out from the apex. The area as a whole is also somewhat 

 arched, not quite level; at the posterior edge it occupies about one-fifth of the entire circumference of the shell. The 

 height (from the apex to the level of the edge of the shell) is about two-thirds of the breadth. Shell smooth, shiny, 

 with fine concentric growth lines visible even on the area, where, however, they are feebler, 

 apex where the shell, as above mentioned, has been removed, 

 there is found a furrow which on the area is parallel with 

 its nearest side edge, and in the opposite direction runs 

 obliquely toward the side. In the direction from the apex 

 down toward the anterior edge the shell is quite arched. The 

 species differs from Acrotreta sodalis von Soebach, as known 

 from examples from the forchhamm£ri zone, by its greater size, 

 its distinct keeled area, and the more subcentral position of 

 the apex, which again causes the arch to be less raised. 



Observations. — This shell is nearest to Acro- 

 treta seelacU (PL LXXVII, figs. 3, 3a). It differs 

 in having the apex farther back from the posterior margin and in the strong median ridge on 

 the false area. It is very close to Acrotreta carinata Moberg and Segerberg. 



Formation and locality. — Passage beds between the Cambrian and the Ordovician: (310) Ceratopyge lime- 

 stone (zone 4), at Ottenby, on Oeland Island; and (309) Ceratopyge limestone {zone 4), at Fogelsang, 5 miles (8 km.) 

 east of Lund, Province of Malmohus; both [Moberg and Segerberg, 1906, p. 66] in Sweden. 



^ Acrotreta claytoni Walcott. 



Plate LXXIV, figures 3, 3a-b. 



Acrotreta claytoni Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 583. (Described as below as a new species.) 



The material representing this species does not give sufficient data for the preparation of a 

 detailed description. The only ventral valve is depressed by compression in the shale; it is 

 transversely broad oval in outline, with the posterior margin nearly straight for a distance of 



Figure S! .—Acrotreta drcularis Moberg and Segerberg. 

 view. B, Posterior ^^ew. C, Side view. Ali X 8. 

 The specimen represented is from Locality 309, the Ceratopyge 

 limestone at Fogelsang, Province of Malmohus, Sweden. The fig- 

 ures are copied from Moberg and Segerberg f 1906, PI. III. figs. 4a-c]. 



