Difflugia lobostoma Leidy, 1879
Diagnosis: test ovoid or subspherical, circular in cross section. Test composed of small and medium pieces of quartz, bounded together by organic cement. Aperture with variations of the opening, trilobed or fourlobed or irregular with indistinct lobes, bordered by an irregular raised ridge of small particles. Lobes range from squarish to curved. Aperture often with an indication of a collar. Nucleus with a number of small nucleoli. Zoochlorellae may be present.
Dimensions: my measurements 136-192 µm, nucleus 37-49 µm ; Ogden (1980) 123-186 µm; Penard (1902) 140-170 µm.
Remarks: based on the shape and structure of the test this species is hard to separate from Difflugia gramen. The key character is the shape and structure of the nucleus, which is completely different from those of D. gramen. In practice however this character is not very useful, as you have to press the cover slip to break the test. If your are lucky, you can see the nucleus - if it comes out of the broken test!
Difflugia lobostoma is larger than D. gramen.