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|Challengeron diodon: |
Shell ovate, slightly compressed, with a single straight conical spine on the aboral pole, half as long as radius. Peristome short and broad, collar-shaped, about twice as broad as long , and half as long as the radius, obliquely inclined over the mouth, with two divergent straight teeth, which are conical and longer than the shell-radius; beyond each tooth a large ovate hole in the wall of the peristome.
Dimensions: Diameter of the shell 0.08 to 0.1, length of the teeth 0.06 to 0.08.
Habitat: South Eastern Pacific, Station 298, depth 2225 fathoms.
|Challengeron nathrostii: |
Shell ovate to subspherical, with a single spine at the apical pole, as long as the radius of the shell or longer. Diameter of the mouth half as long as the diameter of shell. Structure: regular hexagonal alveoli, quincuncially arranged in obliquely decussating rows (3 in 0.01 mm). Peristome finely punctuate, with two long and pointed, hollow, almost parallel horns, and below each of them a triangular or ovate hole.
Diameter of the shell 0.06 to 0.08 mm. The nearest relative is C. diodon from the south-eastern Pacific Ocean.
On more weakly developed (probably young) specimens, the characteristic byspines are wanting. It is therefore certainly most practical to do as Borgert has done and consider as one species, Challengeron diodon, C. heteracanthum, and C. nathorsti CL.
Not particularly frequent and as a rule very sparse, always in deep water.
Distribution: On the west coast of Norway, frequent, but always sparse. Found at a few places in the North Sea (May 1903 at a great depth, according to L. 18) and the northern part of the Atlantic up to the west of Spitzbergen, in deep water. According to Borgert 1. c.  also found in the Labrador Current, the more southerly part of the Atlantic and in the Mediterranean. According to Haeckel, Challengeron diodon has been found in the south eastern part of the Pacific, at a great depth.