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.' 



* See B. Thompson, Midland Naturalist, vol. x. p. 121. 



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