600 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



aperture is at the apex of the pedicle valve and quite readily seen in 

 several specimens, a character in strong- contrast with the minute 

 aperture at the apex of A. se/imalensei. Shell formed of several thin 

 layers or lamelkv that show very plainly where the thin outer layer is 

 exfoliated. Surface marked l)y striiv and lines of growth that are ver}^ 

 distinct on some shells and less soon oOhers; the concentric stria^ occur 

 on all the lamella? of the shell and on the inner surface; fine radiating 

 striffi are to be found on the inner surface. The cast of the interior 

 of the pedicle valve shows the presence of a large apical callosity and 

 unusually large foraminal tube and main vascular sinuses on each side 

 of the visceral cavity. The cardinal scars are well defined in both the 

 pedicle and brachial valves. Casts of the interior of the brachial valve 

 show a strong median ridge, central scars, and fairl}' well-defined main 

 vascular sinuses. Professor v. Seebach describes the surface as having 

 minute warts on it. I find numerous fragments of the shell of Acro- 

 t/iele (jtanulata associated with Acrotreta sociaHs, and it may be that it 

 was the surface of this shell that he mistook for that of the species he 

 was desci'ibing. No known species of Acrotrda has such a surface. 



A large pedicle valve has a diameter of 5 mm, and a height of 2.5 

 mm. The average size is about 3 nun. in diameter. 



This species belongs to the A. Muhconlca group of species, with a 

 broad false area and well-defined median groove. Its surface is more 

 strongly marked by concentric strife than an}' other species of the 

 genus, and the shell is also thicker. 



Formation and locality. — Middle Cambrian, Paradoxides: oJandicus 

 zone, Oland, Island of Borgholm, Sweden. 



ACROTRETA SUBCONICA Kutorga. 



Acrotreta suhconica Kutorga, Uber die Siphonotretae Verhandl., Russisch-Kaiserl., 

 mineralogischen Gesell., 1848, p. 275, pi. vii, figs. 7a, b, c, b^ c^ 



Original description. — Strongly conical; the deltidium-like furrow narrow and 

 plainly impressed. Innumerable growth-wrinkles run on the whole surface of the 

 shell horizontally, and make deflections only in the furrow, the convexity i:>f whicli 

 is turned toward the base of the cone. 



Height of the cone, 0.014?; length of the l)ase or of the ventral valve, 0.012?; 

 breadth of the same, 0.015?. 



Four specimens, of which one is complete and three are without ventral valve. 

 From the collection of Herr v. Volborth. 



On the specimen with the apex broken off I investigated the surface of the frac- 

 ture, under the microscope with a magniiication of 45 times, and found on it two cruri- 

 form, shallow impressions similar to those on the casts of the Siphonotretes. Their 

 surface was polished and wdth impressions of growth folds; and between the extrem- 

 ities of the crura in the neighborhood of the area-like hinge surface, a columniform 

 fragment of the mold of the siphon. From this it proceeds that the broken-off tip, 

 just as the beak of the Siphonotretes, was solid and contained a cylindrical siphon. 



Observations. — Through the courtes}^ and permission of Dr. F. 

 Schmidt, Dr. F. Huene kindly sent me the tvpes of this species, which 

 he had been studying. One of them preserves the outer shell at the 



