CONSPECTUS OF DIATOMAOE.fi. 25 



and again at the middle of the frustule, as seen at fig. 5. Suppose, 

 now, the sutural zone to become wider at one margin of the frus- 

 tule when it passes from fig. 1 to fig. 2, and widest at the middle 

 of the dorsal surface, it would now appear as in fig. 6, which is the 

 end view of a Cymbella. We should find the frustule under action 

 of gravity lying upon one of its valves when allowed to fall freely, 

 and so it would present itself generally in side view. Imagine now 

 an excessive development of the sutural zone, as in fig. 7 (which 

 is an end view, as in fig. 6), the frustule would no longer rest upon 

 one of its valves, as in figure C, but upon the expanding connecting 

 zone between the two dorsal surfaces, and generally we look down 

 upon the frustule from c through to d, in which case both median 

 lines, e and/, would be in view, and if the median line incurved 

 towards c, as it does in many species of the Amphora?, we would 

 now have the view presented as figure 9." 



The connecting zone being almost invariably presented to view, 

 lias led some authors to found specific differences upon it, but cha- 

 racters must be received with caution, and very possibly are value- 

 less. Gregory's group of what he calls complex forms, are frustules, 

 in which from some cause abortive attempts at division have taken 

 place. 



Professor Smith divides the genus into two sections. 

 A with median line incurved. 

 B with median line not incurved. 



These are again divided and subdivided into twenty-three classes. 

 He has only introduced one new species, with the following specific 

 characters : — 



A. undata, n.s. Doubly lyrate and somewhat angularly con- 

 stricted at the middle. Nodule distinct, valves with several longi- 

 tudinal lines, inflexed like the margins of the frustules, and con- 

 verged at the ends ; inner margin of valves slightly curved ; 

 connecting zone with longitudinal lines. Marine. Length *0017, 

 breadth -00U75, transversely striate, strias fine, about 55 in. "001, 

 dry frustule, straw colour. 



In brackish grounds, near New Haven, Conn. 



The number (121) of previously described species he reduces to 

 7G. The following are the forms retained : — 



