Ch. XV.] 



EOCENE PEEIOD. 



201 



Fig. 179. 



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

 has covered its case with shells of a small 

 Plano?'bis. In the same manner a large 

 species of caddis-worm, which swarmed in the 

 Eocene lakes of Auvergne, was accustomed 

 to attach to its dwelling the shells of a small 

 spiral univalve of the genus Paluclina. A 

 hundred 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 

 tlie, as in fig. 180, at right angles one to the other. When we consider 



Fig. ISO. 



Larva of recent Phryganea.* 



a. Indusial limestone of Auyergne. 



5. Fossil Paludina 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 Phryganem hved on the spots where their cases 

 are now found ; they may have multiplied in the shallows near th6 

 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 which the tubular cases of 

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

 occurring in the interior 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 indusiae 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 phragmites, but chiefly the 

 former. 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 Cypris 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 figured is P. rhombica, Linn. 



