Ch. XV.] EOCENE PERIOD. 201 



Fig. 179. 



the annexed cut, whicli belongs to a species very abundant in England, 

 has covered its case with shells of a small 

 Planorhis. In the same manner a large 

 species of caddis-worm, which swarmed in the 

 Eocene lakes of Auvergne, was accustomed 

 to attach to its dweUing the shells of a small 

 spiral univalve of the genus Paliidina. A 

 Larva of recent Phryganea.* j^^ndred of these minute shells are some- 

 times seen arranged around one tube, part of the central cavity of which 

 is often empty, the rest being filled up with thin concentric layers of 

 travertin. The cases have been thrown together confusedly, and often 

 lie, as in fig. 180, at right angles one to the other. When we considei 



Figr. ISO. 



a. Indusial limestone of Auvergne. &. Fossil Fahidma magnified. 



that ten or twelve tubes are packed within the compass of a cubic inch, 

 and that some single strata of this limestone are 6 feet thick, and may 

 be traced over a considerable area, we may form some idea of the count- 

 less number of insects and mollusca which contributed their integuments 

 and shells to compose this singularly constructed rock. It is unnecessa- 

 ry to suppose that the Phryganece lived on the spots where their cases 

 are now found ; they may have multiplied in the shallows near the 

 margin of the lake, or in the streams by which it was fed, and their 

 cases may have been drifted by a current far into the deep water. 



In the summer of 1837, when examining, in company with Dr. Beck, 

 a small lake near Copenhagen, I had an opportunity of witnessing a 

 beautiful exemplification of the manner in Avhich the tubular cases of 

 Auvergne were probably accumulated. This lake, called the Fuure-Soe, 

 occurring in the inteiior of Seeland, is about twenty English miles in 

 circumference, and in some parts 200 feet in depth. Round the shallow 

 borders an abundant crop of reeds and rushes may be observed, covered 

 with the indusise of the Phryganea grandis and other species, to which 

 shells are attached. The plants which support them are the bullrush, 

 Scirpus lacustris, and common reed, Arundo phrogmites^ but chiefly the 

 formxcr. In summer, especially in the month of June, a violent gust of 

 wind sometimes causes a current by which these plants are torn up by 

 the roots, washed away, and floated off in long bands, more than a mile 

 in length, into deep water. The Oypris swarms in the same lake ; and 

 calcareous springs alone are wanting to form extensive beds of indusial 

 limestone, like those of Auvergne. 



* I believe that the British specimen here ficjured is P. rhomhica, Linn. 



