390 THE PALEONTOLOGY OF MINNESOTA. 



(.Strophomena septiita. 



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 S. 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. incurvata, 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. C. L. Herrick, E. O. Ulrich and the writers 

 Mns. Reg. Nos. 345, 6*6, 4936, 6795, 6798. 



STROPHOMENA RUGOSA (Rafinesque Ms.) Blainville. 



PLATE XXXI, FIGS. 4 and 5. 



1825. Strophomena rugosa (RAFiNESQUE) BLAINVILLE. Malacologie et Conchyliologie, vol. i, p. 513, 



pi. LIU, flgs, 2, 2a. 



1827. Strophomenes rugosa DEFKANCE, Dictiounaire des Sciences Naturelles, vol. i, p. 151 and atlas. 



1847. Leptcena planumbona HALL. Palaeontology of New York, vol. i, p. 112, pi. xxxi, flgs, 4a to 4e. 



1850. Strophomena rugosa KING. Permian Fossils, p. 103. 



1862. Strophomena planumbona HALL. Geology of Wisconsin, vol. i, p. 54, flg. 7. 



1873. Strophomena (Itemipronites) planumbona MEEK. Paleontology 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 MILLER. Ibidem, vol. ii, p. 45 



1877. Streptorhynchus planumbonus MILLER. American Palaeozoic Fossils, p. 134. 



1878. Streptorhynchus elongata MICKELBOROUGH and WETHBRBY. 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. n, flgs. 13, 14. 



1881. Strophomena planumbona WHITE. Tenth Report, Indiana State Geologist, p. 115, pi. n, 



flgs. 13, 14. 



1883. Streptorhynchus planumbona HALL. Second Annual Report New York, State Geologist, 



pi. xxxix, flgs. 15-17; pi. XLII, flgs. 8, 9. 



