473 ANIMAL NATURE OF DIATOMEiB. 



to the formation of a diaphragm? a circumstance in 

 which the essential difference between this and the pre- 

 ceding species seems to reside. 



In the fossil flour of Santa Fiore, besides the T. 

 amphicephala, we find many fragments which cannot all 

 be referred to the two preceding species. The vittse 

 seem to have disappeared in all, owing to the absence of 

 colouring matter, and the under-mentioned conditions of 

 the shield are clearly seen in all. 



It is justly observed, byHassall, that there is a difference, 

 in the mode of connection, between the two common 

 species {jlocculosa, fenestratd). In the second, there is a 

 distinct cushion (cussinetto), in the form of a hinge, 

 connecting one frustule to another. In the first, the 

 angles are nearer together, the hinge is wanting, and 

 there remains in its stead a sort of detached border, 

 showing that the very fine external membrane has been 

 lacerated, yielding to the excessive distension, but 

 persisting partially to maintain the union, and without 

 retracting itself, as it must do, in the other species, to 

 originate the hinge. 



This diversity of condition appears to me in accordance 

 with the different period of development at which the 

 deduplication occurs in the two species, as well as with 

 the other considerations already mentioned. I think 

 that we ought to take this fact into account, as also 

 serving to support my opinion, as to the nature of this 

 external membrane. 



Besides the two living species, and the fossil one just 

 mentioned, Kiitzing describes another, discovered by 

 Lenormand ; and enumerates three from America, the 

 discovery of which is due to Ehrenberg. 



56. Terpsinoe. — bacilli tahulati adnati, obsolete 

 stipitati, demum semisoluti et isthno concatenati, vittis 

 transversalibus abbreviatis mar<jinalibus {non perviis) 

 capitatis in latere secundario nodosi. 



If we imagine a series of frustules of Tabellaria joined 



