162 ON THE CHOKING UP OF HARBOUHS. 



curious structure appears to have been built up, and illustrated his des- 

 criptions by highly magnified drawings of the manner in which the 

 grains are arranged in the fibrous skeleton. Spicula were of rare oc- 

 currence in this specimen ; when observed, they were embedded in the 

 external coating of the cartilaginous fibres ; they are short, and com- 

 paratively thick in proportion to their length, decreasing very slightly 

 from the middle to near the points, and are terminated acutely, but 

 somewhat abruptly. 



The third species is the Spongia fragilis of Montagu, or Duseideia 

 fragilis of Dr. Johnston's manuscript. 



It is massive, variable in form, of a dull ochreous yellow colour, and 

 has the surface asperated by the projection of fibres, which contain nu- 

 merous grains of sand, embedded in a manner similar to those described 

 in treating of the last species. There are also other fibres, which are 

 tubular in their structure, containing few or no grains of sand, but an 

 abundance of spicula, remarkable for their great variety in form and 

 size. No spicula were found in the fleshy matter of the sponge, but a 

 considerable number of round or oval bodies were observed, which 

 presented every appearance of being cytoblasts. 



The author concluded his paper by some observations on the present 

 state of our knowledge of the structure of the Spongiada, and noticed 

 certain changes that will become necessary in their systematical arrange 

 ment, when our information regarding their structure is more matured. 



Drawings of the species described, and the various forms of spicula 

 contained in their structures, illustrated the paper. 



XXXIV. OBSERVATIONS UPON THE IMPORTANT PART WHICH MICRO- 

 SCOPIC ORGANISMS PLAY IN THE CHOKING UP OF THE HARBOURS 

 OF WISMAR AND PILLAU J ALSO IN THE FORMATION OF THE MUD 

 WHICH IS DEPOSITED IN THE BED OF THE ELBE, AT CUXHAVEN, 

 AND UPON THE AGENCY OF SIMILAR PHENOMENA IN THE FORMA- 

 TION OF THE BED OF THE NILE, AT DONGOLA, IN NUBIA, AND IN 

 THE DELTA OF EGYPT.* 



By M. Ehrenlerg. 



DURING the course of the year 1839, M. Ehrenberg made special re- 

 searches upon the form of the mud- banks in the harbour of Wismar in 



* Report of a memoir read to the Berlin Academy of Sciences, in March 1841, 

 extracted from Professor Jameson's Edinb. New Phil. Journ., Oct. 1841, p. 386. 



