One of the most remarkablo fossils assumed to be distinctive of a particular geological horizon, and which, from its very striking outline, most readily impresses itself upon the mind, is the Rhynchonella acuta of the Lias-marlstone, a Brachiopodous shell common at Stinchcombe, Churchdown, and other localities of this district, and well known elsewhere. Having paid considerable attention to the class to which it belongs, I have long abandoned the common practice of placing in the cabinet only those specimens which chance to accord with the forms figured and described as typical. Instead of doing this, I have selected, as good examples, those which manifestly have not been crushed or injured prior to their entombment and petrifaction; and these I have arranged in series illustrative of specific development.

This mode of procedure has taught me that the species under consideration assumes forms varying from that under which it is most generally known; and it has led me to believe that several so-called species of various authors are, in reality, mere varieties of this.

All who have attentively studied the numerous Terebratulidæ of the Cotteswolds will have experienced the difficulty of assigning satisfactorily certain anomalous forms, occurring in beds ranging vertically from the Pisolite, or even lower, to the Cornbrash, to such well-established species as Terebratula maxillata, T. perovalis, T. globata, or T. intermedia, and will remember the remarkable varieties of individual character presented by other species, as, for example, T. plicata, T. simplex, T. fimbria, and T. carinata, sufficiently striking when studied in solitary examples, but, in an extensive series, not suggestive of good and stable specific differences.