S32 Report of the State Geologist. 



Siphonotreta (?) Minnesotensis. 



Plate 2, figs. 9, 10. 



Siphonotreta (9) Minnesotensis, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, 

 p. 177, pi. iv, figs. 37, 38. 



Shell subovate in outline. Pedicle-valve more convex than the 

 brachial, slightly flattened along the median line, sloping with 

 equal convexity toward the lateral and anterior margins. Fora- 

 men apical (?). Brachial valve depressed-convex, somewhat ele- 

 vated about the umbo. Surface covered, in the umbonal region, 

 with fine, anastomosing and gently undulating concentric lines, 

 which, in the later portions of the shell, are finely granulose or 

 serrated ; at about one third the length of the shell coarser var- 

 ices of growth appear, between which the finer lines are retained. 

 Surface covered with hollow spines of various sizes, which appear 

 to have been most closely set over the umbonal region of the 

 pedicle- valve. Here, where the growth-lines are absent, the 

 spine-bases in the original specimen are large and all of about 

 the same isize and are disposed without order. Over the other 

 portions of the shell the spines are set along the edges of the 

 varices, small and large being indifferently mixed. The bases 

 of the spines make annular swellings on the interior of the 

 valves The length of the original specimen is 15 mm., width 

 12 mm. 



Trenton limestone. Minneapolis, Minnesota. 



Orbiculoidea (Schizotreta) ovalis. 



Plate 2, 'figs. 12, 18. 



Orbiculoidea (Schizotreta) ovalis, Hall. Palaeontology of N. Y., vol. VIII, 

 pt. 1, p. 177, pi. ive, figs. 4, 5. 1892. 



Shell subelliptical in outline. Yalves with apices situated a 

 little behind the center. Lo ^er valve with the apex erect or 

 inclined slightly forward. External foraminal groove narrow, 

 extending for about one-half the posterior radius of the valve. 

 Surface about the foramen convex, but elsewhere slightly 

 depressed in its slope from apex to margin. Upper valve more 

 elevated than the lower, apex inclined backward, posterior slope 

 gently concave. Surface marked by elevated nearly equidistant 

 concentric striae. Shell-substance thick, the inner laminae show- 

 ing fine radiating'lines over the anterior region. Length of the 



