Naviculaccu.'] ixfusouial animalcules. 451 



verse witli respect to tlie bands ; when iiicomplete, we have eon- 

 catenate, when complete, single forms ; it is seen to commence 

 beneath the transparent siliceous lorica. The members of this 

 genus are found, both in salt and fresh water, parasitic on Algae, 

 in all parts of the world ; several species have also been met 

 with fossil. 



This genus, Achnanthes, is a member of, and gives the designation 

 to the family Achnanthece of Kiitzing ; which also comprises the 

 genera Achnanthidhcm and Cymlosira. As a family it most nearly 

 resembles Striatcllece, but the latter w^ant the umbilicus of the 

 Aclmantheee. 



"VYe add the following particulars from Mr. Ealfs : — 



" The ui)per margin of the frastules of Achnanthes is convex ; the 

 lower one concave. In some species the lateral portions are turgid, 

 the central portion looking like a band between them ; in others they 

 are flat, and do not enter into the fi'ont view. The superior 

 lateral siu'face differs from the lower one in the absence of the 

 central transverse pellucid line, which is present in the lower, and 

 by its terminaton forms the pimctum seen in the front view. The 

 mode of growth in this genus resembles that in Isthmia, except 

 that the finxstules finally separate without cohering at their angles." 



" Achnanthes differs from all other Dlatomaeea, except Striatella, by 

 its stipitate, flag-like fronds ; tmd from StriatelJa it may be known 

 by the absence of internal siliceous plates ; and even a solitaiy 

 ft'ustide of this genus may be distinguished from that of any other, 

 by its curved form, and by the punctum in the middle of the lower 

 margin." (Ann. Nat. Hist. v. 13, p. 490.) 



Achnanthes longipes. — Large, striated, curved (excised), at the 

 middle on the ventral aspect ; convex dorsally ; ends obtuse, rounded; 

 end view, elliptic-oblong ; pedicle thick, from two to five times the 

 length of the lorica ; supporting but few indi^^duals. Transverse 

 striae ten in l-1200th. Length (/. e. breadth of band) l-570th. to 

 1-1 20th. The central portion of each fi-ustule appcai-s like a band 

 between the turgid lateral portions. Common in sea-water. 



A. hrevipes. — Resembles the preceding generally ; differing, 

 however, in the end view, which is suddenly and shortly acuminate, 



