Bacillaria.] THE INFUSORIA. 173 



and at the places where these creatures are connected 

 together it is effected by means of soft processes pro- 

 truding through these openhigs. Excepting the genera 

 Navicula, and one or two others, they never separate 

 spontaneously into single individuals, but always adhere, 

 forming polypi-like masses of greater or less extent ; hence 

 it is that the term imperfect self-division has been applied 

 to their mode of propagation. 



As this family is of such vast importance in a geological 

 point of view, and naturalists are so divided as to the 

 natural position of its members, it will be desirable to 

 enter into a few general particulars respecting it, to which 

 I shall append an abstract of Dr. Meyen's views, they 

 being unknown to the English reader. 



The composition of the lorica or shell of these creatures 

 is various, and may be separated into two kinds ; the first 

 containing silica, either pure or in combination with the 

 oxide of iron, forming a silicate of iron ; or secondly, those 

 in which silica is entirely absent; the lorica has then a 

 membranous or parchment-like (structure) texture. It is 

 remarkable that in no case has lime been found to enter 

 into their composition. In some genera the lorica is sur- 

 rounded by a soft gelatinous variously-formed envelope 

 or induvium. The shape of the lorica is various, but such 

 as entirely to inclose the animalcule (hence it is termed 

 urceolate), except the parts where it is united with others. 

 When such is the case, and it is surrounded on all sides, 

 the lorica is composed of two or more shells, or pieces, 

 termed valves, which are usually dish or cup-shaped, and 

 often fluted or grooved. Those in which silica enters into 

 the composition have usually a round or a prismatic four- 



