1890.] MICKOSCOPICAL JOUKNAL. '279 



parently having but one hoop, whereas there are really two, and In no 

 figure is the prime point indicated — namely, that one valve with its 

 hoop or band fits into the opposite valve with its hoop or band. So 

 that the student who relies upon those drawings will never get at the 

 truth. 



Nave's Handbook is just as faidty in both text and drawings. Thus 

 he says, "' It may help to guide the young botanist if he keeps in mind 

 that the ' sides ' are the silicious membranes, which, from their enclosing 

 the contents of the plant, are more appropriately named ' valves,' while 

 the * front' is the frame or hoop, as it is generally termed, which binds 

 the flinty surfaces together. And then he shows two valves and the 

 hoop by itself, and while this figure is a Navicula he uses it to illus- 

 trate Blddulphta and Tricei-atiuiii. Now, that is not the correct 

 description of either Biddiilphla or Tricei'atium or even of some 

 species at least of Navictila. The student gets the idea that the two 

 valves are held together by a ring which fits over the margins of the 

 valves and which is a piece by itself. But it is not so in the cases 

 mentioned. And many of his figures of the front view are incorrectly 

 drawn, while again in no case is the important point indicated, that one 

 valve fits into the other valve. 



Hogg, too, is just as misleading in his figures, and while he says 

 scarcely anything about the hoop or band in the text, what he does say 

 is incorrect. 



Carpenter states the matter correctly. He says, " As soon as the 

 valves begin to undergo any increase, they separate from one another ; 

 and by the silicification of the cell- membrane thus left exposed, a pair 

 of hoops is formed, each of which is attached by one edge to the adja- 

 cent valve while the other edge is free. One of the valves is always 

 older than the other (this, however, I question, for when the frustule 

 comes from a sporangium both valves, unless I am very much mistaken, 

 are of the same age), and the hoop of the older valve partly encloses 

 that of the younger, just as the cover of a pill-box surrounds the upper 

 part of the box itself. As the newly formed cell increases in length, 

 separating the valves from one another, both hoops increase in breadth 

 by additions to their free edges ; and the outer hoop slides ofi'the inner 

 one until there is often but a very small overlap (p. 328, 6th edit.) Con- 

 trast the description of the process in the Micrographic ; " the two valves 

 sepaiate from one another, remaining connected by the simultaneous 

 gradual widening of the hoop. The history and ultimate fate of the 

 hoop seems to be variable. 



" Perhaps the most remarkable development of the silicified hoop oc- 

 curs in Biddulphia^ Isthitiia^ and similar torms : the new half frustules 

 formed inside the hoop of these genera slip out from it like the inner 

 tubes of the outer case of the telescope," which is not so in the case of 

 either Biddulphia or Isthmia. Carpenter, I say, states the matter cor- 

 rectly. Dr. Wallich had done so before him, and Prof. Hamilton 

 Smith confirmed Dr. Wallich's view. But their articles are not in the 

 hands of the ordinary student. J. D. Cox mentions the two hoops of 

 Isthmia^ but adds a third which he asserts overlaps them both at the 

 line of junction. Carpenter, however, asserts that there is no such third 

 hoop,* and there can be no doubt that he is right. But while Carpen- 



* p. 328, note. 

 I 



