of Microscopical Org-anisms. 335 



Fig. 20. Navicula . This form is doubtfully 



referred by its observer to N. bacillum, E. The sides are 

 nearly straight, the ends rounded, the striae transverse, close, 

 and very faint. The median line has a faint line running 

 through its centre, which is prominent near the ends, but 

 grows suddenly faint at a short distance from them. 



Fig. 21. Navicula . This form is probably K 



lineolata, Ehr. = JV. serians, K., but as neither the descrip- 

 tion nor the figures given of this species by these writers 

 is sufficiently minute, it is difficult to recognize. In the 

 present specimen, the longitudinal lines are distinct with 

 low powers, and with the highest powers may be seen to 

 be separated by transverse lines, resolvable into dots. 

 Neither Ehrenberg nor Kiitzing allude to any transverse 

 lines whatever, but they may not have sufficiently searched 

 for them. 



The next form (Figs. 22 and 23) I have been unable to 

 find, and only retain the form in the plate that it may be 

 identified if found at any future period. 



Fig. 24. This form, I have no doubt, is Navicula g-as- 

 trum of Ehrenberg and Kiitzing = N. placentula, Ehr. The 

 striaB are faint, close, and slightly radiant, resolved into 

 dots by I'g in. objective. 



Fig. 25. Biddulphia tenuis, L. W. B., nov. sp. ? Shell 

 excessively thin, quadrangular, with the whole surface de- 

 cussately punctate ; puncta under low powers appear like 

 lines; lateral view elliptic, with acute, slightly produced 

 apices. 



The form represented in Figs. 25 and 26, for which I have 

 substituted the name of Biddulphia instead of Dcnticella 

 originally assigned to it, is probably, I think, a variety of 

 Zi/g-oceros Mohiliensis, B. = Biddulphia Bailcyii of Smith. 

 The two, however, were probably carefully compared by 

 their discoverer, and I therefore hesitate to abandon the 

 distinction drawn by him. In the pencil sketch from 



