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W O R K S OF d a r w i n : v o l u m e 1 4 anterior ones, which must have come from the carinal half of the whorl, but the exact position of which I cannot tell. These latter larger latera are thin, and considerably convex; they are transversely elongated, and, in their longer axis, are rather more than half the length of the terga of the same individual. In shape they may be almost compared to one valve o f a Donax; being subtrigonal, with the two upper sides unequal in length, and with the third side arched and protuberant at a point about one-third o f the entire length of the valve from the narrower end: to this protuberant point, a well-defined ridge runs from the apex / of the valve; internally (i), the lines of growth round the upper margins show that the two upper sides and the apex overlapped freely the valves on each side of them. This valve, as stated in the preliminary remarks, has naturally been always mistaken for a scutum; but the manner in which it has overlapped other valves (as shown by the internal view), on both its upper margins, and the entire absence of all hollow for the adductor muscle prove that this view is quite erroneous.