1 14 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



with the species all the varieties in which the number of cusps, or rather 

 lobes, to the mouth, varies from three to half a dozen or more. A variety 

 of the same species, with a five-lobed mouth, is figured and described 

 by Fresenius, and referred by him to the D. oblonga of Ehrenberg.* The 

 reference is, however, improbable, for Ehrenberg, in speaking of the Difflu- 

 gia tricuspis of Carter, remarks that Fresenius regards it as D. oblonga, but 

 adds that he had not perceived a three-lobed mouth to the latter,f from 

 which it may be suspected that no lobes whatever were present, as appears 

 to be the case, if we may found an opinion on the figures of that form in 

 the ' Infusionsthierchen.' 



The shell of Difflugia lobostoma is commonly ovoid, with the mouth 

 situated at the narrower pole. Less frequently it is oval or nearly spherical, 

 or the ovoidal form may be more or less prolonged at the narrower pole, so 

 as to assume a sub-pyriform shape. The mouth may truncate the shell, or 

 it may not interfere with the curvature of the oral pole, or this may more 

 or less protrude so as to form a short neck or rim. 



Usually the mouth is trilobed, or is bordered with three rounded lobes 

 or sinuses separated by angular points, and resembles the trefoil opening 

 employed in architectural decoration, as seen in figs. 1, 3, pi. XV. The 

 number of lobes of the mouth may, however, vary from that given to half 

 a dozen or more. Next in frequency to the trilobate mouth, according to 

 my experience, is the six-lobed mouth, as seen in figs. 10, 12, and after this 

 comes the quadrilobate mouth, as seen in figs. 5, 8, 16. 



Large specimens, with a many-lobed mouth, approximate" Difflugia 

 corona, and indeed I have observed so many of intermediate condition in 

 all respects, that there is little doubt that the two species merge into one 

 another. 



The shell of Difflugia lobostoma is usually composed of angular quartz- 

 sand, in common with other species of the genus. Barely it is com- 

 posed of diatoms, or diatom-like plates, together with fragments of others, 

 sometimes with variable proportions of quartz-sand. Small shells occa- 

 sionally occur composed of chitinoid membrane with quartz particles 

 incorporated. Individuals in which the shell is composed wholly of quartz- 

 sand are represented in figs. 1-15, pi. XV. 



* Abh. Senckenb. Naturf. Gesells. ii, 1856-8, 225. 



* Abh. Ak. Wis. Berlin, 1871, 238, 239. 



