ARCHER, ON A NEW SPECIES OF MICRASTERIAS. 2-43 



lobed, the basal and middle lobes dicbotomously divided ; the 

 lobes closely approximate throughout^ widening the entire 

 way from the base^ the ultimate subdivisions not being 

 attenuate, their end lobes narrow, and the endochrome con- 

 taining scattered, rounded, large granules, and frequently 

 retracted from the margin of the frond, so as to leave a 

 hyaline border. M. rotata and M. denticulata differ in the 

 following particulars : — The former is notably a larger species 

 . than the latter ; in front view, in the former, the basal lobes 

 are twice, the middle lobes thrice, dichotomous — in the latter 

 both basal and middle lobes are but twice dichotomous, thus 

 making in tlie former the ultimate subdivisions of the middle 

 lobes eight (and this additional dichotomy makes itself appa- 

 rent at an early stage in the growth of the young segments), 

 and making in the latter the ultimate subdivisions of the 

 middle lobes but four only (in the former, therefore, the 

 middle lobes are wider in proportion than in the latter) ; in 

 the former the ultimate subdivisions of the basal and middle 

 lobes are acutely bidentate — in the latter they are truncato- 

 emarginate, with more or less rounded, sometimes subacute, 

 angles ; in the former the end lobe is greatly more distin- 

 guishable in character and appearance from the others than 

 in the latter ; in the former the outline of the end lobe is 

 narrowed below, inflated above the base, again contracted, 

 and again dilated and spreading at the extremity, — in other 

 words, more or less of a narrow, campanulate outline (the 

 adjoining upper margin of the middle lobes closely approxi- 

 mating thereto by a corresponding outline) — in the latter 

 the end lobe is simply cuneate, with the sides somewhat con- 

 cave ; in the former, the end lobe is distinctly exserted beyond 

 the general periphery of the frond — in the latter it is not 

 exserted, its outer margin being continuous with that of the 

 frond ; in the former the angles of the end lobe are pro- 

 duced, divergent and bidentate — in the latter its external 

 angles are bluntly rounded ; in the former the central 

 emargination of the end lobe is not so deep as in the latter ; 

 in the former, the endochrome is more dense within the end 

 lobe, especially at both of its margins, than elscAvhere in the 

 frond, giving it a turgid appearance, and rendering the cam- 

 panulate outline more striking — in the latter it is not so. 

 In the side "siew in the former (fig. 9), the frond is stouter 

 and more turgid than the latter (fig. 6), its sides less con- 

 cave. In the end view the former is stouter, and the 

 outline is broadly fusiform, with a solitary central, slightly 

 elevated, rounded projection at each side (fig. 10) — in the 

 latter the outline is more slender, and is lanceolate, with 



