98 The American Geologist. February, 1897 
consists. In these respects the O. multisectd Meek appears like a re- 
sultant from the variation noted in O. rogata. 
A theory to explain the origin of the double median plication of the 
ventral valve and of the single dorsal median plication, can be construc- 
ted as follows: that the variation of the median plication with paired 
offsets to the same with alternating offsets, and to one with the first 
one of the latter paired with the median itself, as seen in O. rogata, has 
gone still farther owing to early dwai-fing of all individuals, and the 
completely paired median plications of the ventral valve of O. nmUisecta 
resulted. (See figs. 29 a-d pi. V.) 
This character having been transmitted by inheritance is retained in 
even these larger shells which returning favorable ccmditions have pro- 
duced. Corresponding to the change in the ventral valve an inverse 
change in the dorsal obtained. The dorsal valve is less convex, the 
ventral more convex than the same in the average of O. rogata. 
Variation in O. viultisecta is noticeable, in particular that one of the 
paired median plications is sometimes the smaller, or, on the contrary, 
one or both at a late stage of the shell's growth give off a email branch 
into the fun-ow between the two. Really dwarfish specimens of this 
species aj'e probably not found. 
ORTHIS MEEKI S. A. Miller. 
PI. IV, fig. 2i to 29. 
Orthis eni%aernt(i Mesk, (1873), Palaeontology of Ohio, vol. i, plate 8> 
tig. 2a-g, (uon Hall). 
Orthis meeki S. A. Miller, (1875), Cincinnati Quarterly Jour, of Sci., 
vol. II, p. 20. 
Orthis. {Dalmanella} festn.dinaria Hall, (1802), Palaeontology of New 
York, vol. VIII, pt. 1, pl.5B, fig. 32-37. 
Specimens of O. meeki of smaW size, but apparently immature ones, 
have been collected at Cincinnati, Ohio, at about 2{X) feet elevation above 
the Ohio river. Larger ones have been received from Oxford and Le- 
banon, Ohio, and Madison , Indiana. The vertical range may therefore 
be said to include the whole upper half of the Cincinnati series, at 
least. It is not yet known in the area of the Maquoketa series. It is 
associated with O. iiiultisecfa Meek. 
Orthis meeki is larger, hence more loosely plicated than O. multiseetn 
Meek, but the plication is, in general, alike. Ortlds -meeki is less con- 
vex, and has a stronger median fold and sinus than the other, and is 
easily mistaken for O. emacerata Hall, from which, however, it has a 
quite different plication and surface ornamentation. The interior of the 
valves, so far as seen, are very much like O. maltisecta, but the thick- 
ening of the shell makes deeper muscle scars on the dorsal valve, and a 
narrower muscular ai-ea of the ventral valve. I have found no transi- 
tional connection between Ortiiis miiltisecta Meek and O. meeki S. A. 
Miller, and although such have been believed, by authors, to exist, 
still the same informants confuse also O. meeki with O. emecevata and 
O. corpidenta. 
