370 THE PALEONTOLOGY OF MINNESOTA. 



I Sc'lii/oc'rania Hlosa. 



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. tit.). 



Type : Orbicula ? filosa Hall. 



Species of this genus are found in the Trenton, Utica and Hudson River groups 

 of America. S. helderbergia Hall, from the Lower Helderberg, and S. superincreta 

 Barrett, of the lower Oriskany, are other American species. 



SCHIZOCKANIA. FILOSA Hall, 

 PLATE XXIX. FIGS. 20-31 



1847. Orbiculaf 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. xm, flgs. 21, 22. 



1875. Trematis (t) filosa MILLEK. Cincinnati Quart. Jour. Sci., vol. ii, p. 15. 



1875. Sehizocrania fitosa HALL and WHITFIELD. Paleontology of Ohio, vol. ii, p. 73, pi. I, flgs. 12-15. 



1892. Sehizocrania filosa HALL. Palaeontology 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 stria3 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 



