454 DESCiiirTiON of IPoli/gastrica. 



curved ; end view lanceolate, striate ; small plates, composed of two 

 or three frustnlos, supported by a short pedicle. Were found abun- 

 dantly on grains of the beach sand, below high water mark, at Fort 

 Brooke, Truspa It is possibly a species of Hijaloseira, but requires 

 further study. 



Genus AcnwANTHiniUM, (Kiitz.) — Indi\'iduals solitary, or binatc, 

 free (not adnate) ; primary side (dorsum) linear, bent (genuflexed.) 



A. Microcepli(ilnm. — Secondary side lanceolate ; each end capitate. 

 Length l-1680th. Marine. Common among other Diatomacea. 

 (?. 17, f. 15.) 



A. delicatulum. — Secondary side ventricose ; extremities prolonged. 

 Length l-1680th. In brackish water. (P. 17, f. 16.) 



Genus Amphiteteas, (Ehr.) — Lorica simple, bivalve or multi- 

 valved; quadrate, with four openings on each surface, viz. one at each 

 angle ; seK-division imperfect ; but the chain-like masses which the 

 individuals form, are not gaping, as in some other genera. 



This description differs from that by Mr. Ealfs and Kiitzing^ 

 chiefly in respect of the non-attachment of the members. Ealfs 

 thus characterizes the genus : — 



" Filaments attached by one of the angles of the basal fnistule ; 

 frustules cubiform, rectangular, or, more frequently, with all the 

 angles slightly produced, reticulated, cohering in a zig-zag chain." 

 Along with Biddulphia and Denticella, Amphitetras approaches very 

 near to Fragilnria : it is far removed from Staurastrum, to which it 

 bears considerable external resemblance, (Elu'.) 



Mr. Ealfs observes : " This genus agrees with Isthnia and Bid- 

 duJpliia, in the reticulated structure of the frustrdes ; in the great 

 size of the lateral sui-faces, between which the central portion appears 

 like a band ; in the produced angles, which are situated entirely in 

 the lateral portions, and in cohering at the angles, and thus forming 

 a zig-zag chain. It also agrees with these genera in their peculiar 

 mode of growth ; but it differs from Idhnia, in having all its angles 

 equal, and from both in having a four-sided, and not a compressed 

 figure."— (Ann. Nat. Hist. v. 12, p. 275,) 



Amphitetras is one member of the family A7ri/ul{fera, of Kiitzing, 

 which includes besides, Lithodemiium and Ampliipentraa, and is 



