422 THE PALEONTOLOGY OF MINNESOTA. 



LOrthis (Dinorthis) detli-rtii. 



posterior end of which is drawn inwards, and is transversley striated. When this 

 plate is absent, as is commonly the case, the apical portion of the delthyrium is 

 filled with shell matter which served for the attachment of the pedicle muscle. In 

 Orthis tricenaria the apical plate is always strongly developed and may be flat, con- 

 cave or convex, the latter condition not being common. A small deltidium is also 

 present in Orthis (Dinorthis) pectinella. In all the species of Orthis observed when 

 a pedicle muscle is present a deltidium is absent; but where this plate is developed 

 the muscle is rudimentary. This evidence leads the writers to the conclusion that 

 the pedicle muscle is attached to the bottom of the valve in the apex of the delthy- 

 rium when the deltidium is wanting, but when it is developed the muscle is then 

 more or less attached to the deltidium. 



ORTHIS (DINORTHIS) DEFLECTA Conrad, sp. 



PLATE XXXII, FIGS. 2430. 



1843. Strophomena deflecta CONRAD. Proceedings of the Academy of Natural Sciences of Phila- 

 delphia, vol. i, p. 332. 



1843. Stroptiomena recta CoNEAD. Ibidem, vol. i, p. 332. 



1847. Leptcena deflecta HALL. Palaeontology of New York, vol. i, p. 113, pi. xxxiB, fig. 5. 



1847. Leptcena recta HALL. Ibidem, p. 113, pi. xxxiB, flp. 6. 



1859. Strophomena deflecta HALL. Twelfth Report, N. Y. State Cabinet of Natural History, p. 70 



1859. Strophomena recta HALL. Ibidem, p. 70. 



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



1889. Streptorhynchus deflectum MILLER. North American Geology and Palaeontology, p. 378. 



1892. Plcesiomys deflecta HALL. Palaeontology of New York, vol. viii, pp. 197, 222, pi. vA, flgs. 28-34. 



1892. Plcesiomys recta HALL. Ibidem, pp. 197, 222. 



Original description: "Semioval, superior [ventral] valve slightly concave 

 deflected at the angles, the other valve reflected; radii very closely arranged, 

 prominent, subequal, minutely crenulated; inferior [dorsal] valve slightly depressed 

 in the middle; cardinal area wide; superior margin of the concave valve rather 

 elevated. Breadth, half an inch." 



Shell semioval or subquadrate in outline; concavo-convex or strophomenoid in 

 form. Hinge-line two thirds, or as long as, the greatest transverse diameter of the 

 shell; width of cardinal areas variable, in some specimens comparatively wide, 

 widest in the ventral valve, and disposed at a right angle to each other; delthyrium 

 of both valves broadly triangular and partly covered by a convex deltidium. Sur- 

 face marked by fine, equal, sharply rounded striae, which increase in number by 

 interstitial addition and rarely by bifurcation. From 120 to 135 along the anterior 

 margin of adult examples. Striae and intermediate depressions crossed by numerous, 

 elevated, thread-like, concentric lines and a few well marked minor imbrications, 

 the latter indicating periods of rest in shell growth. 



Ventral valve carinate medially in the posterior third, with the lateral and 

 anterior portions concave; the latter part is the stronger and forms a broad and 

 undefined mesial sinus; greatest elevation at the beak. Cardinal area flat, broadly 



