390 
THE PALEONTOLOGY OF MINNESOTA. 
LStrophomeua septaUi. 
Strophomena septata W. and S. 
PLATE XXX. FIGS. 1-3. 
1892, April 1. Strophomena septata W. and S. American Geologist, vol. ix, p. 285. 
This species appears to be a local development of S. trentonensis W. and S., and 
as far as external characters are concerned, no distinguishing features can be 
pointed out. Compared with S. rugosa, a still closer resemblance, both externally and 
internally is seen. However, when the interior is shown it can be separated readily 
from both by the strong mesial septum of the ventral valve. This originates between 
the diductor scars and continues to increase in strength to near the anterior margin, 
where it often coalesces with one or two of the vascular ridges. In S. trentonensis 
the thickening of the interior near the anterior margin of the ventral valve is obso- 
lete or entirely undeveloped, constituting another distinguishing feature between it 
and 8. septata. The cardinal process of both species is also more elevated, while the 
rostral thickening. upon which it rests is developed less strongly than in -S. rugosa, 
these parts being much as in S. incurvata. S. trentonensis and S. septata never attain 
the thickness of shell nor the abundance of individuals of S. rugosa. The septum of 
the ventral valve in *S'. septata will also distinguish it from S. iflcurvata, in addition 
to its smaller size and comparatively greater width than length. 
Formation and locality. — Common in the upper third of the Trenton shales at St. Paul, Minneapolis 
and near Rochester, Minnesota. 
Collectors.— G. L. Herrick, E. O Ulrich and the writers 
M^^s. Reg. Nos. 345, 676, 4936, 6795, 6798. 
Strophomena rugosa (Rafinesque Ms.) Blainville. 
PLATE XXXI. FIGS. 4 and 5. 
1825. Strophomena r wflrosa (Rafinksque) Blainville. Malacologie et Conchyliologie, vol. i, p. 513, 
pi. Liii, flgs. 2, 2a. 
1827. Strophomenes rugosa Defrance, Dictionnaire des Sciences Naturelles, vol. i, p. 151 and atlas. 
1847. LeptoRna jJlanumbona Hall. Paheontology of New York, vol. i, p. 112, pi. xxxi, figs. 4a to4e. 
1850. Strophomena rugosa King. Permian Fossils, p. 103. 
1862. Strophomena planumbona Hall. Geology of Wisconsin, vol. i, p. 54, fig. 7. 
1873. Strophomena {Eemipronites) planumbona Meek. Palaeontology of Ohio, vol. i, p. 79, pi. vi, 
flgs. 3a-3h. 
1874. Streptorhynchus (Strophomena) elongata James. Cincinnati Quarterly Journal of Science, 
vol. i, p. 240. 
1875. Hemipronites planumbona Milleu. Ibidem, vol. ii, p. 45 
1877. Streptorhynchus planumbonus Miller. American PalcBozoic Fossils, p. 134. 
1878. Streptorhynchus elongata Mickelborough and Wetherby. Journal Cincinnati Society of 
Natural History, vol. i, p. 76. 
1880. Strophomena planumbona White. Second Annual Report, Indiana Bureau of Statistics and 
Geology, p. 483, pi. ii, flgs. 13, 14. 
1881. Strophomena planumbona White. Tenth Report, Indiana State Geologist, p. 115, pi. ii, 
flgs. 13, 14. 
1883. Streptorhynchus planumbona Hall. Second Annual Report New York, State Geologist, 
pi. XXXIX, flgs. 15-17; pi. XLn, flgs. 8, 9. 
