FOSSIL SHELLS AND FISH. 421 



Gasteropoda are rare in the lias ; the most remarkable is 

 the extinct genus Pleurotomaria, of which several species are 

 found. Magnificent specimens of a large species, (P. Anqlicafy 

 were found near Bredon. Another species known as Motella 

 is special to the marlstone. 



The Cephalopoda are very abundant in the liasic beds. 

 Seventeen species of Belemnites occur in the lias of France, 

 and nearly as many are found in those of England. The 

 internal shells of a genus of the family Loligidae, have been 

 found in the lower lias of Cheltenham ; * in the same 

 stratum three species of Nautilus, N.striatus, N.semistriatus, 

 and N. intermedius, are found, and in the Marlstone one 

 species, JV. inornatus, is special to it. D'Orbigny has figured 

 and described one hundred and forty-two species of Ammo- 

 nites, from the lower, middle, and upper lias of Prance, and a 

 great number of the same species are found in the lias of 

 Gloucestershire. The different groups of Ammonites contain 

 species that characterise the several zones of the liasic beds. 

 Thus, in the inferior zone of the lower lias shales, we find 

 A. bisulcatus, A. obtusus, A. subarmatus, A. liasicus, A. tortilis, 

 A. Conybeari, A. Birchii. In the superior zone of the lower 

 lias shales, A. planicosta, A. Boblayei, A. Valdani, A. Henleyi. 

 In the Marlstone of Dumbleton, A. spinatus, A. Engelhardti, 

 A. margaritatus, and A. heterophyllus. In the upper lias 

 shales, A. bifrons, A. serpentinus, A. annulatus, and A. ovatus. 



"We have already stated that with the lias, an important 

 modification in the structure of fishes was introduced ; many 

 genera of the Ganoid order with equal lobed tails are found 

 in the lias of Lyme and Whitby, as Tetragonolepis, Dape- 

 dium, Amblyurus, Semionotus, and Lepidottis, but for further 

 details relating to this class we must refer to the great work 

 by Agassiz on fossil fishes, to our chapter on palaeontology, 

 and to the lists of British fossils already cited for the genera 

 and the localities from whence they are obtained. 



A thin seam of insect limestone lying near the base of the 

 lower lias beds, and exposed at Wainlode and Westbury cliffs, 

 on the banks of the Severn, has yielded Insects belonging 

 to several families of Coleoptera, Orthoptera, and others, 



* We have recently found internal shells of Loligidee in the upper lias of 

 Dumbleton. 



