Chap. 12.] GEOLOGY, BOTANY, &o. 251 



The most numerous of the mountain limestone fossils are the 

 brachiopoda ; almost the lowest family of themoUusca.' They possess 

 a bivalve shell, differing from that of the cockle (Cardium) in that the 

 valves are unequal, and correspond to the dorsal and ventral aspects 

 of the animal. In the cockle and other Lamellibranchs the valves of 

 the shell are equal and correspond to the sides of the animal. These 

 constitute a higher family of the mollusca. Among the genera of the 

 brachiopoda found in the Mountain Limestone the most frequent are 

 Productus (Plate II., fig. 2), Spirifera (Plate II., fig. 3), Terebratula 

 (Plate II. fig. 7), Orthis (Plate II., fig. 6), Ehynchonella (Plate II., 

 fig. 9), Strophomena, Chonetes, and Leptsena ; most, if not aU, of 

 which may be found in the quarry at Thomley. 



A few lamellibranchs, gasteropods, and cephalopods are also found, 

 such as Conocardium, Euomphalus (Plate II., fig. 1), Murchisonia, and 

 Goniatites (Plate 11. fig. 5). 



The brachiopods of Carboniferous times gradually died out, and at 

 present are represented in British seas by two species. They were as 

 gradually replaced by the higher types, the lamellibranchs and 

 gasteropods, which now number in the same area over three hundred 

 marine species. Another interesting organism found at Thornley is 

 the Trilobite (Plate II., fig. 8). In ages much more ancient than the 

 Carbonifeious, the order Trilobita was represented by a great many 

 genera and species. The order gi-adually dwindled away and finally 

 died out in the Carboniferous period, where it is represented by four 

 genera and a few dwarfed species, three of which have been found at 

 Thornley. 



From the meagre development of the Mountain Limestone in this 

 district we can form no conception of the loveliness of the scenery 

 where it is well developed. It is in Yorkshire and Derbyshire that it 

 may be seen in all its magnificence. There, the eye is delighted with 

 picturesque hills fissured by deep, rugged ravines, and pierced by huge 

 caverns or grottoes whose gloomy interiors are furnished, as if by 

 enchantment, with grotesque imitations of animals and other objects, 



^It is worthy of note that the mass of to have been gradually developed with 



this ancient fauna consisted of animals each succeeding deposit. 



of a low type. The higher types appear 



