88 



BECREATIVE SCIENCE. 



feasor Smith called it " connecting mem- 

 brane," an excellent descriptive term, with 

 the one fatal objection of too great length. 

 "We shall adopt the elegant and euphonious 

 classical word of " cingulum" — a zone, or 

 band, since proposed for it by a later writer. 

 The cingulum is always in two portions, 

 and must, from the necessities of the case, be 

 so. This may be rendered more clear by 

 rig. 5. 



Fig. 5. — Biddulphia aurita, in outline, ccc, the cin- 

 gulum ; two new valves, faintly indicated by dotted 

 lines, are seen in process of formation within it. 



This double character of the cingulum, 

 with the enclosure of that portion formed in 

 connection with the one valve by that of the 

 other, has not hitherto been clearly pointed 

 out. Its use as an admirable provision for 

 protection of the newer, to whatever extent 

 the older valves may be pushed apart, is 

 evident, and also that the regulated access 

 of water needful for life may be thus allowed. 



7lO 



Tia, 6.— a, 5, c, d, represent different stages of infold- 

 ing of the primordial utricle, with division of the 

 nucleus ; at e this is complete, and at / the new 

 valves are represented as fully formed ; n, the 

 nucleus. 



Folding in of the primordial utricle, or lining 

 membrane (Fig. 6), is the next step, and by 

 it the new valves are secreted, thus complet- 

 ing the process. 



" Multiplication by division " in this way 

 is probably very rapid. "With a due degree 

 of patience and care an observer might 

 watch it through all its stages, and furnish 

 sound data for calculations on the time occu- 

 pied, instead of the guesses we at present 

 have. On the assumption that the entire 

 process occupies in each case twenty-four 

 hours, it has been calculated that " we 

 should have, as the progeny of a single 

 frustule, the amazing number of one thou- 

 sand millions in a single month."* 



The terms " side view " and " front view" 

 are used to distinguish the different aspects 

 of a frustule ; the former when the valve is 

 fully seen, as in the supposed case of looking 

 directly down on the centre of our snuff-box ; 

 the latter, where the cingulum is seen with 

 merely the edges of the valves, as on turning 

 the box edgewise. 



The term Diatom (from two Greek words 

 signifying breaking or dividing into two 

 parts) may be considered as having reference 

 to the mode of increase by self-division, or, 

 as suggested by Dr. Carpenter, to the readi- 

 ness with which in Diatoma, one of the 

 commonest forms, the highly siliceous frus- 

 tules, held together merely by a little gela- 

 tinous hinge at the opposed corners, break 

 up ; whence the ordinary name of the family, 

 Brittleworts, 



Fig. 7. — Diatoma vulgare, in filament, represented as 

 growing attached to a blade of grass. To show the 

 character of the frustule the endochrome has been 

 omitted. 



The true position of the Diatomacea; has 

 for many years been a subject of controversy, 



* Smith, " Synopsis of Br, Diatomacese," Introd,p.x3cr. 



