786 



THE WONDERS OF GEOLOGY. Lect. VIII. 



swarm in our seas, also existed in the most remote periods 

 of which we have any traces of organic life. 



Species of the living genus Chiton, have recently been 

 found in the Silurian rocks of Ireland.* 



But it is the brachiopodous mollusca, that constituted 

 the great mass of the population of the Silurian seas, and 

 were rivalled only in numbers and variety by the ex- 

 tinct crustaceans, the Trilobites. The ancient types, as 

 Atrypa, Calceola, Leptcena^ Pentamerus, and Spirifer, 

 are associated with species of Lingula, Orthis, and Tere- 

 bratula, — genera which still exist. Twenty-five species 

 of Terebratula, and thirty- eight species of Orthis, and 

 seven of Lingula, have been obtained from the British 

 strata alone. 



The Potsdam sandstone, the most ancient fossiliferous 

 rock of New York, is in many places divided into laminae 

 by the remains of innumerable shells of the genus Lingula. 

 They are in such profusion as to form black seams like 

 mica, and are accompanied with another small placunoid 

 shell, which is also associated with a small species of Lin- 

 gula in the lowest beds of the English Silurian series in 

 Brecknockshire. Here, then, in the most ancient term of 

 organic life is a shell belonging to a genus not extinct, and 

 very like a species still living. J 



In fact, throughout the palaeozoic deposits the brachiopo- 

 dous mollusca abound ; and hence we may infer the immense 

 development of this family in the ancient seas. Nearly 

 half the Silurian shells figured by Sir R. I. Murchison in 

 his Geology of Russia, belong to brachiopoda. The genera 

 Orthis, Spirifer, Leptaena, &c. are the most common forms 



* See a notice of a fossil Chiton from the Silurian rocks of Ireland, 

 by Mr. Salter, Geological Journal, vol. iii. p. 48. 



f Three species of Leptama have recently been discovered in the 

 Lias of France. 



X Mr. Lyell's Travels in America, p. 157. 



