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 A HISTORY OF NORTHAMPTONSHIRE local type than those of the formations last mentioned, being in fact those common to the English Lias generally. Of the topmost beds of the Upper Lias' Ammonites communis is a characteristic shell, while lower down it is replaced by A. serpentinus, and still lower (in the transition beds to the Marlstone) by A. acutus. The so-called ' fish and insect bed ' at the base of the Upper Lias takes its name from the remains of the two groups of animals which form its special feature. In the top- most beds of the Middle Lias, or Marlstone, Ammonites acutus and A. holandrei are characteristic fossils ; below the true rocky bed of the Marlstone A. spinatus occurs in the softer beds ; while below these latter the zone of A. margaritatus is reached. It has been observed that the last-named zone is noteworthy for its numerous bivalves, while that of A. spinatus abounds in brachiopods, the topmost transition beds contain- ing an equally noticeable percentage of gastropod molluscs. The Lower Lias has been divided into a number of zones, each re- presented by distinctive species of ammonites ; but these are too numerous to mention on the present occasion. In addition to these ammonites and several kinds of belemnites, the Lower Lias is especially characterized by several peculiar bivalves, such as the Lias oyster, Ostrea liassica, the curved gryphite, Gryphcea incurva (the 'devil's toe-nail' of the quarry- men), the great Lima gigantea, the massive Hippopodium ponderosum, the swan's-foot mussel, Avicula cygnipes, and Cardinia listeri and other species of the same genus. Very interesting, too, is Spirifer walcotti, the last sur- vivor of a group of Paleozoic brachiopods. Finally, the crinoids, or stone-lilies, are represented by Pentacrinus tuberculatus, the detached joints of the stems of which are familiar to quarrymen as 'star-stones.' 46
 * See B. Thompson, Midland Naturalist, vol. x. p. 121.