288 FORAMINATED POLYTHALAMOUS SHELLS. 



SECTION VIII. 



FORAMINATED POLYTHALAMOUS SHELLS. 



Mummuliies. 



If the present were a fit occasion for such minute in- 

 quiries, the investigations of the various known species of 

 Microscopic shells would unfold a series of contrivances 

 having relation to the economy of the minute Cephalopods 

 by which they were constructed, not less striking than those 

 we have been examining in the shells of extinct Genera 

 and species of larger Cephalopods. M. D'Orbigny has 

 noticed from 600 to 700 species of these shells, and has 

 prepared magnified models of 100 species, comprehending 

 all the Genera.* 



The greater number of these shells are microscopic, and 

 swarm in the Mediterranean and Adriatic. Their fossil 

 species abound chiefly in the Tertiary formations, and have 

 hitherto been noticed principally in Italy. (See Soldani, as 



* M. D'Orbigny, in his classification of the shells of Cephalopodous 

 Mollusks, has established three orders. 1. Those that have but a single 

 chamber, like the shell of the sepia and horny pen of the Loligo. 2. Poly- 

 thalamous shells, which have a siphuncle passing' through all the internal 

 chambers, and which terminate in a large external chamber, beyond the 

 last partition, such as Nautili, Ammonites, and Belemnites. 3. Poly- 

 thalamous internal shells, wiiich have no chamber beyond their last parti- 

 tion. 



Shells of this order have no siphuncle, but tiie chambers communicate 

 with each other by means of one or many small foramina. On this distinc- 

 tion he has founded his Order Foraminiferes, containing five families and 

 fifty-two genera. 



It may be necessary to apprize the reader that doubts have been enter, 

 tained as to the cephalopodous structure of some of these minute multilocu- 

 lar shells ; and that there are not wanting those who attrib ute to them a 

 different organization. 



