422 THE PALEONTOLOGY OF MINNESOTA. 
[Orthls (Dinorthisi deflecta. 
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) pedinella. 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 XXXIl, FIGS. 34-30. 
1843. Strophomena deflecta CoNUAD. Proceedings of the Academy of Natural wSciences of Phila- 
delphia, vol. i, p. 332. 
1843. Strophomena recta Conkai). Ibidem, vol. i, p. 332. 
1847. Leptcena deflecta Hall. Pakeontologj- of New York, vol. 1, 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 Palajozoic Fossils, p. 134. 
1889. Streptorhynchus deflectum Millek. North American Geology and Palieontology, p. 378. 
1892. Ploesiomys deflecta Hall. Pahcontology of New York, vol. viii, pp. 197, 222, pi. vA, flgs. 28-34. 
1892. Pltesiomys recta Hall. Ibidem, pp. 197, 222. 
Orifjinal description: "Semioval, superior [ventral] valve slightly concave 
deflected at the angles, the other valve reflected; radii very closely arranged, 
prominent, subequal, minutely crenulated; inferior [dorsalj 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. Stria? 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 mssial sinus; greatest elevation at the beak, ('ardinal area flat, broadly 
