128 DESMIDS OF THE UNITED STATES. 



This species diflfers from all others of this genus in the form 

 of the cell, often twice as long as broad. Fig. 2 is very near 

 a form which Reinsch denominated M. angulosum. It is pro- 

 portionately shorter than the typical form of Ralfs, but other- 

 wise so near, it scarcely admits of separation. 



Section VI.— Lobes horizontal, attenuated, bidentate. 



M. LATICEPS, Xord., 1869. {M. disputata, Wood, 1872.) Plate 

 XLI, figs. 4. 5. 

 Quadrangular, about as long as broad, snbpinnatisected, 

 sinuses acute; terminal lobe nearly as long as the basal lobe; 

 both the lobes strongly attenuated ; the polar lobe into an 

 acute point and the other into an acutely bidentate apex ; 

 ends rounded or emarginate. 

 Diameter 160-212 yu. 



This species made its first appearance from Brazil and was 

 described by Nordstedt in 1869 in a scientific journal of 

 Sweden. Wood gave the same plant a new name a few years 

 later. The former has a claim of priority, hence should 

 stand. The plant varies considerably in size, and in the form 

 of the terminal lobes ; some are more rounded than others. 

 It has been found in a large number of States. 



M. osciTANS, Ealfs. Plate XXXVII, figs. 3, 4. 



Cells of nearly equal length and breadth : the end lobe 

 separated from the adjoining lobe by a rounded or acute- 

 angled sinus : lobe horizontal, conical, their extremities 

 usually bidentate. One form has the ends of the terminal 

 lobe acute. The end lobes are much shorter and narrower 

 than the others. Transverse view fusiform ; membrane 

 punctate. 



Diameter 1.50-160 /(. 



Bailey reports this species from Florida and Rhode Island, 

 but makes the measures much less. I have specimens from 

 Florida, Massachusetts and New Jersey. They were of the 

 given measures. 



M. PiNXATiFiDA. Kg. . Ealfs. Plate XLI. fi.i^s. 7. 8. 9. 



Ends straight ; semi-cells deeply constricted, lobes hori- 

 zontal, more or less fusiform with apice^ bidentate. 

 Diameter varies from 8.':!-110 ju. 



Somewhat like the preceding, but very mucli smaller, and 

 lobes not so swollen. 

 Much more frequent than M. oscifans. 



Some are found with the basal lobe wide (fig. 9); these I 

 have named, 



