Naviculacea.'] infusorial animalcules. 429 



two species have been frequently confounded, but tbeir habitats arc 

 quite distinct, the present being marine, the next freshwater; 

 the Hippocampm, too, is a shorter and stouter species, and its stria; 

 more dehcate." 



h. Freshwater species. 



Pleuiiosigma attenuatum = Navicula Ilippocampus (Ehr. ?) — ^Valvc 

 elongated, lanceolate, obtuse ; flexure moderate ; striae very distinct ; 

 colour purpHsh-brown. Length l-120th. Breadth 1- 1000th. Lon- 

 gitudinal stria; l-25,000th apart. Transverse striae l-40,000th (P. 20, 

 f. 11 ; fig. 12 a young specimen; fig. 13 a front view, showing self- 

 division; fig. 18 a portion of fig. 11, magnified 3200 diameters.) 

 Common in streams and ditches. 



P. ?acMS^/r.-^— Valve lanceolate, much attenuated towards the acute 

 extremities; colour pale brown. Length l-144th. Breadth 1-1 100th. 

 Longitudinal transverse striae l-45,000th apart (P- 20, f. 14.) 



P. Spencerii. — Valve lanceolate, slightly attenuated, obtuse ; 

 flexure shght; colour very pale brown. Length l-270th. Breadth 

 l-2000th. Striae l-50,000th apart. Not uncommon in ditches. 



Genus OcoNTiDnm. — (de Brebisson.) — Frustules quadrangular on 

 secondaiT side, transversally, striated, lanceolate ; closely conjoined in 

 a biconvex band. 



Ehrenberg has described the fonns belonging to this genus, some 

 among Fragilaria, others with Bacillaria (Diatoma.) 



Odontidium belongs to the family Fragilariem, and exhibits an 

 alliance with. Ilimantidium, Diadesmis, and also with Meridion. The 

 concatenation of the frustules, so as to form a band or filament, is 

 well marked in the present genus. Its forms are most common in 

 cold alpine springs. 



0. mcsodon = Fragilaria mesodon (Ehr.) — (P. 16, f. 24.) 



0. turgidulum = Fragilaria turgidida (Ehr.) — (P. 16, f. 25.) 



0. glaciale. — Filaments short and generally curved ; segments 

 mostly trapezoid, margin with many dentations ; on secondary side 

 elliptic-oblong, rather obtuse ; striae five to six in l-1200th. Among 

 Confervae, Glacier of the Rhone. v 



0. hgemale = Fragilaria striata (Ehr.) and F. hjemalis (Balfs.) 



0. piiinatum = Fragilaria pinnata (Ehr.) 



