This is Mazon Monday post #13.  

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Belotelson magister is the most common crustacean/shrimp fossil in the Mazon Creek biota.  They can be found fairly readily and can range from a partial, to a molt, and sometimes even a full body.  Most of the shrimps found in Pit 11 are Belotelsons.  It was described way back in 1886 in material from what was probably the Mazon River.

Example fossils are below.

Molts

Here is the text from Creature Corner.  It appeared in the ESCONI bulletin in June 1986.

Belotelson magister is the most abundant arthropod at Pit 11.

Two thirds of all the Essex crustaceans found are this species. Yet less than 25 years ago, only three specimens were known since their first recorded discovery in the Mazon Creek area during the 1880s.

Literally·”thousands” of concretions containing parts of complete specimens of this species have been found in the Essex (marine environment) portion of Pit 11. The most common remains, by far, of Belotetson magister are cast-off exoskeletons (molds).

A puzzle underlies this last statement. It has been noted that concretions containing parts and pieces of this shrimp are extremely common. Concretions bearing portions of other Mazon Creek shrimp are rather uncommon. In ensuing years concretions enclosing Belotelson magister have been discovered in Francis Creek shale at other locations in Illinois.

Belotelson magister was rather large compared to other Carboniferous crustaceans. The carapace on some specimens measures two inches in length. It had well developed walking legs, large antennae, and a large fan-tail. General form, body structure, and walking legs suggest a creature with living habits not unlike the modern crayfish. Bottom dwelling, an opportunistic feeder, scavenger or carnivore, to suit the occasion.

— Andrew A. Hay