THK PALEONTOLOGY OF MINNESOTA. 
[Pcbizocrania Slosa. 
convex, with the beak marginal. The interior bears a pair of strong posterior 
adductor scars, lying close together in the umbonal region; their outline is elongate- 
ovate, indicating a progressive increase in size, and they frequently appear to be 
divisible into anterior and posterior elements. In front of them, at about the center 
of the valve, are the small and faint anterior adductor impressions. A low median 
ridge extends from the apex to beyond the center of the valve. External surface 
marked by elevated striae radiating from the beak. 
"Substance of the shell composed of perlaceous calcareous laminae which con- 
stitute the most of the shell. The inner layers appear to be corneous. All are 
impunctate?" (Hall, op. cit). 
Type : Orhicula ? filosa Hall. 
Species of this genus are found in the Trenton, Utica and Hudson River groups 
of America. S. helderhergia Hall, from the Lower Helderberg, and S. superincreta 
Barrett, of the lower Oriskany, are other American species. 
SCHIZOCEANIA. FILOSA Hall, 
PLATE XXIX, FIGS. 29-31 
1847. Orhicula? filosa Hall. Palaeontology of New York, vol. i, p. 99, pi. xxx, flgs. 9a-9d. 
1863. Trematis filosa Billings. Geology of Canada, p. 159, flg. 126. 
1873. Trematis filosa Hall. Twenty-third Rep. N. Y. State Cab. Nat. Hist., pi. xiii, flgs. 21, 22. 
1875. Trematis (?) filosa Miller. Cincinnati Quart. Jour. Sci., vol. ii, p. 15. 
1875. Schizocrania filosa Hall and Whitfield. Palaeontology of Ohio, vol. ii, p. 73, pi. i, flgs. 12-15. 
1892. Schizocrania filosa Hall. PaliBontology of New York, vol. viii, pt. i, p. 143, pi. ivG, flgs. 22-30. 
Original description: " Orbicular; one valve more or less convex; apex marginal ; 
surface radiated with numerous fine elevated thread-like striae which are more or 
less prominent, depending on exfoliation of the shell; intermediate striae coming in 
between the others as they recede from the beak, but the striae are not bifurcate." 
This species was subsequently more fully described by Hall and Whitfield from 
material obtained at Cincinnati, Ohio. The description is as follows; "Shell orbic- 
ular, or very slightly ovate, the beak of the upper or free valve [dorsal] projecting a 
little beyond the limits of the circle, giving a somewhat greater diameter along the 
median line than in a transverse direction. Free valve moderately convex, the 
central region being the most prominent. Attached valve [ventral] discoid, very 
thin, deeply and broadly notched on the posterior side; the notch not extending 
quite to the center of the valve; occupying nearly one quarter of the circumference 
of the valve on the outer margin ; border of the notch thickened, especially at the 
base, which is rounded and the center marked by a slightly projecting point. Inter- 
ior of the free valve [dorsal] marked by two proportionally large, elongate, ovate, 
diverging muscular prominences [posterior adductor scars], leaving corresponding 
pits on the casts of the shell, or on exfoliated specimens; situated just below the 
