ZONE OP ARIETITES OBTUSUS. 49 



5. The Zone of Arietites obtustjs, 



Synonyms. — " Marston-Marble," Sowerby, 'Min. Concb.,' Suppb Index, vol. i, 1812. 

 " Indurated marl and limestone-beds," De la Beche, " Section," &c., ' Geol. Trans.,' 

 2nd ser., vol. ii, pi. iii, 1823. " Turnerthone," Quenstedt, 'Flozgeb. WiJrttembergs,' 

 p. 157, 1843. "Ammonite-bed (Lower Lias)," Murchison, 'Geol. of Cheltenham,' 

 2nd edit., p. 42, 1845. "Sable d'Aubange " (partie infer.), Dewalque et Chapuis, 

 'Luxembourg,' p. 12, 1853. " Die Schichten Aqs, Ammonites obtusus," Oppel, 'Jura- 

 formation,' p. 50, 1856. " Gres de Virton " (partie infer.), Dewalque, 'Lias de 

 Luxembourg,' p. 48, 1857. "Zone oi Ammonites obtusus,^' Wright, 'Quart. Journ. 

 Geol. Soc.,' xvi, p. 404, 1860. 



Gloucestershire and Warwickshire. — The beds constituting this zone are well developed 

 in the Vale of Gloucester, and were exposed in cutting the Bristol and Birmingham 

 Railway, near Bredon, from whence the best collection of the fossils from this zone 

 in the Midland Counties was obtained. The rocks consist of dark-grey and bluish shales 

 and clays, with irregular and inconstant beds of dark-grey argillaceous limestone, the 

 shales being in part nodular and laminated, the clays thick and tenacious, and the nodular 

 portions of the shales very fossiliferous. Several of the Arietites obtusus and Arietites 

 stellaris had their shells well preserved, and the outer layers of the same were adorned with 

 numerous parallel, longitudinal, spiral lines, consisting of punctuated elevations, which 

 extend along the sides, dip into the depressions, and rise on the central elevation of the 

 siphonal area. This ornamentation is limited to the external lamina of the shell, as no 

 impression of it is left on either the nacreous layer or the mould. I have rarely found these 

 punctuated lines so well preserved as in the specimens I have figured of Arietites obtusus, 

 from Bredon and Lyme, and in the remarkable specimen of Arietites stellaris, from 

 Lyme, preserved in the original Sowerbyan Collection now in the British Museum. So 

 seldom is this specific shell-structure observed that many palaeontologists deny its 

 existence in these species, but after the figures I have given all doubt upon the subject 

 must be removed. In Warwickshire this zone forms part of the Cardinia-bed and 

 contains some very fine specimens of Arietites obtusus, Ar. midticostatus, Ar. Brookii, 

 and Ar. Sauzeanus, d'Orb. 



Dorsetshire. — At Lyme Regis the Obtusus-zone attains a considerable thickness, and 

 is well shown in the coast-section. The strata rise on the shore about half a mile west 

 of Charmouth, and consist of thick beds of dark marls, which rest upon the table-bed 

 formed by Broad Ledge. The lower part of the marls contains numerous compressed 

 Aegoceras Birchii, Sow., and layers of nodules forming cement-stones. Above these 

 succeed shales and clays, thin bands of limestone, and thick beds of shale and marls with 

 mudstones. Above these again are inconstant bands of limestone containing septaria, 

 in which gigantic examples of Arietites obtusus, Arietites stellaris, and Arietites Brookii 

 are found. The following section shows the relative position of these beds. 



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