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Show 83_ " UNIO CRISTOXENSIS, Meek. " Shell under medium size, transversely ovate, thick, and strong, moderately convex; anterior outline rounded; posterior more narrowly rounded, and more prominent below than above the middle; basal margin semi- ovate; flank usually with a alight flattening or very faint concavity, extending from the umbonal region downward ; beaks depressed, and placed about half- way between the middle and the front; surface with more or less distinct marks of growth, but without costte, tubercles, or other ornamentation; hinge rather strong; anterior teeth more or less furrowed, that of the left valve deeply bifid, and that of the right sometimes a little emarginated; lateral teeth apparently of moderate length; muscular scars deeply impressed, particularly the anterior, which are very close to the anterior margin, and usually have one or two or more little irregular denticula-tions just above, directly under the anterior division of the cardinal tooth. " As near as can be determined from the fragmentary specimens, the dimensions of one of the largest would be about as follows: Length, 1.65 inches; height, 1.10 inches; convexity, 0.70 inch. " The only specimens of this species found are very imperfect: and, from that fact and their general similarity to Tertiary forms, of which many much better specimens have been brought in from the Far West, I paid but little attention to them when they were sent to me by Professor Cope without any indications of their age. I wrote to him that the form here described resembled U. Haydeni, from the Bridger group ( Tertiary) of Wyoming; but, as a caution against too hastily adopting this suggestion as a settled conclusion, I added, ' you can readily understand, however, how very difficult it would be, in this genus, to identify allied living species from the examination of mere battered and broken odd valves, picked up along the shores of our western rivers/ u Soon after writing as above I received a letter from Professor Cope, informing me that these specimens came from Triassic beds. This, of course, caused me to examine them more closely; and, on doing so, I find that the form most nearly like V. Haydeni in size, form, and proportions, as well as in surface- markings, still differs in having a slight flattening ( already mentioned) down the flanks under the beaks. I think its posterior dorsal outline is also a little more declining, or, in other words, not quite so straight and horizontal as in U. Haydenu I know very little of the hinge of U. Haydeni, but its anterior teeth seem to be different, so far as can be seen, from those of the form under consideration, which also has its valves more thickened internally in front of the middle, and sometimes provided with a low internal ridge, corresponding to the external slight, flattening of the valves. 3 o I i B 8 ? z I - J i r e -. \ •• • & |