Naviculacea. infusorial animalcules. 387 



(wind pipe) ; the rings semicircular ; the extremities alternately 

 conjoined along the centre ; striiie slight, running the length of the 

 tubule. The ends of the tubules have not been seen closed by an 

 opcrculmn. Length 1 -386th. 



Genus Hercotueca, (Ehr.) — Lorica composed of two unequal 

 valves, tui'gid ; membrane of valves continuous, not cellular, gene- 

 rally veined beneath an integument, or divided beneath the free setae, 

 which are permanent, and assume the place of an integument. 

 Hence the corpuscles on the upper and contiguous margin of each 

 valve appear as if crowned and enveloped, as it were, shielded, by 

 the opposed setoe or membranes. 



The forms of this genus generally resemble those of Galliomlla, but 

 are uot spontaneously divisible under a deciduous integument. 



H, mammillaris. — Valves smooth, with the centi'e of the base 

 fringed round (fortified), with twenty simple, opposite setae, extending 

 beyond a series of mammillce, inserted on the margin itself. Diameter 

 1-8 1 6th. Fossil, Bermuda. 



Genus Himantidium, (Ehr.) — It includes several species formerly 

 enumerated with Eunotia. Ehrenberg's characters unlcnown ; those of 

 Kiitadng are, " Lorica, on transverse section, rectangular; transverse 

 striae very fine, and very closely set ; individuals conjoined trans- 

 versely and closely, in the form of bands." All the forms are 

 motionless, and unattached, neither forming fringes nor films on 

 Conferva. They closely resemble Fragilaria, both in form and in 

 their mode of concatenation, and single fr-ustules can be distinguished 

 from the latter only when seen on their ventral surface, and the 

 absence of the central umbilicus thus exhibited. They are of fresh 

 water habit, and pretty generally, though not abundantly dislTibuted. 

 They also occur in the fossil state. 



H. bidcm resembles Eunotia hidens, but developed in chains. 



H. gracile. — BacUli in chains, with the habit of H. Arms; but 

 only half the thickness laterally. Central and Jforth America. 



H. guianense. — Striated laterally ; dilated in the middle ; slightly 

 furrowed, and bidendate on the dorsum : extremities attenuate, 

 slightly reflex. (P. 15, f. 54.) Cayenne. 



H. MonoduH resembles Eunotia Monodon, except in being concate- 



