696 DESCEIPTION OF ENGEAVINOS. 



PLATE XVII. {continued.) 



Figs. 23. Hyalosira rectangula 



24 to 28 inclusive. Cymbella 

 Helvetica 



29. Hyiilosira obtusangula 



30. Sphenella augustata 



31. Sphenella obtusata 



32 and 33. Diadesmis confervaoea 

 34 and 35rt b. Berkeleya Adriatica 

 36. Gomphonema coronatum 



Figs. 37 and 38a, b, c. Homoeociadk 



pumila 

 39 to 42 inclusive, Micromegsl 



pallidum 

 43 to 46 inclusive. Micromega 



Bombycinum 

 47 to 49 inclusive. Homoeocladia 



Martiana 



PLATE XYIIL 



Figs. ^1 to 6 inclusive. Navicula (Pinnularia, Ehr.) viridis From Schleiden'g 

 Principles of Botany, to illustrate the structure of the siliceous lorica; 

 fig. 1, anterior view (venter, Ehr.) " In the middle line are two clefts, 

 terminating at the centre, as well as at the other ends, with a little 

 circular enlargement, more clearly seen in figs. 3 and 5. The rounded 

 spot in the middle, and at the two ends, is not a hole as represented 

 by Ehrenberg. That such a hole is decidedly sometimes not present, 

 is seen in such fragments, as figs. 3 and 5. In the position of the oblique 

 lateral clefts, (striae or costse, Ehr."), the shield consists of two leaves, 

 penetrated by the clefts, which, where both the lamella) touch each 

 other, are somewhat broader, which explains the varying breadth of the 

 clefts according to the alteration of the foci. Fragments in which this 

 structure is clearly represented, may be frequently obtained by crushing 

 the shield (fig. 6.) Fig. 2, a lateral view, showing that the rounded 

 enlargements of the median line are but depressions on the external 

 surface. The double contour, denoting the thickness of the wall, is well 

 seen. This clearly shows that a passage exists from the top to the 

 bottom of the shield, which may be easily confirmed, if the shield, 

 or better still an oblique section of it, be looked at from above ; fig. 5 

 is such a section." 



7 and 8. Cpnatopleura elliptica (Smith.) 



9. Cymatopleura solea. 



10 to 19 inclusive. Closterium Ehrenbergii (Smith.) — Showing the stages in its 

 conjugation, and the formation of the spoi'angia; fig. 10, a single frond 

 in its oi-dinary condition; fig. 11, two fronds approaching to conjugate; 

 fig. 12, conjugating fronds undergoing self-division, the upper showing 

 the protuberances through the torn apices of which the contents of the 

 divided fronds pass into the sporangia ; fig. 13, shows the passage of the 

 eudochrome sac and its contents ; fig. 14, conjugated fronds having 

 perfected their sporangia ; fig. 15, after M. Morren, developmont of the 

 " propagules " into young fronds; figs. 16, 17, IS, 19, from Morren, 

 development of a sporangium into a Closterium with unequal segments. 

 The figures are all magnified 100 times. 

 20 to 26 inclusive. Surirella biseriata (Smith.) — To illustrate the structure and 

 self-division of the lorica ; fig. 24, side view (dorsum or venter, Ehr.) ; 

 figs. 23, a front (lateral, Ehr.) view. The broad median longitudinal 

 band is the connecting membrane of the two valves; fig. 25, an end 

 view of fig, 24 ; fig. 26, transverse section of empty frustule ; fig. 22, 

 silex of connecting membrane after maceration in acid ; fig. 21, aper- 

 tures of costal canals seen in front; fig. 20, view of frustules on the 

 completion of self-division. 



