316 DESCEIPTION OF {Tolygastrica. 



De>"ticella thermdis. — Oblong or subtrapczoid, margin beautifuQy 

 dotted ; secondary side lanceolate ; transverse striae seven to eight in 

 l-1200th. Length l-660th. Hot baths of Abano. 



D. elcgans.— Oblong, with obtuse angles, rather dilated at the 

 centre ; naargin with minute prominences ; secondary side linear 

 lanceolate; transverse striae six in l-1200th. Length 1 -660th. 

 Nordhausen. (P. 16, f. 4.) 



D. ohtusa. — Oblong, large, margin striated ; secondary side lanceo- 

 late; ends obtuse; transverse striae eleven in l-1200th. Length 

 1 -336th. Sweden, Norway, Jutland. 



D. constrida = Navicula (?) constrida, (Ehr.) — Oblong, large, mar- 

 gin dentate ; apices dilated, rounded ; secondary side transversely 

 ribbed, oblong, equal ; each end rounded and truncate ; three to 

 four costae in l-1200th. Length l-216th. Berlin. (P. 16, f. 3.) 



D. undulata = Navicula (?) undulata (Ehr.) — Yery large, oblong, 

 rectangular ; margin dentate ; remarkable by a pair of dotted longi- 

 tudinal flexuose lines ; secondary side elliptic, apices rounded ; 

 transverse costae four in l-1200th. Length l-144th. Berlin. (P. 3. 

 f. 149.) 



Genus LESMOGOxrcTM:, (Ehr.)— (Characters unknown. 

 D. Guianense occurs fossil, a^^iin meteoric dust. From Ehrenberg's 

 figures (P. 24. fig. 13), of this species, Besmogonium would seem 

 to belong to the family Naviculece ; it appears, however, to possess 

 no foramina. 



Genus DiADESMis, (Kiitz.) — Individuals navicula-shaped, conjoined 

 so as to form elongated (biconvex) bands, with one median aperture 

 and two terminal ones, distinct. It is a member of Klitzing's family 

 Naviculece. Ehrenberg has described two of the species with 

 Talellaria, and a third with Navicula. It closely resembles Fragil- 

 laria, from which it differs in its fnistules having a median 

 umbilicus. 



D. confervacea. — Smooth ; length of each individual frustule, i. e., 

 the width, when conjoined in a filament, double that of its shorter 

 diameter ; lateral jimction surfaces lanceolate and acuminate at each 

 end. Length 1 -960th. Among Conferva, Trinidad. (P. 17. f.. 32, 33.) 

 D. Icevis = Tabellaria Iccvis (Ehr.) — Smooth ; length three to four 

 times the breadth. Chili. 



