caudal fin ray count
usually only the principal or main rays are counted, the tiny rudimentary, often procurrent rays are not included. In fishes with branched rays, the principal count is the number of branched rays plus two. These rays are usually markedly larger than the neighbouring ones and originate from the hypural plate. In some fishes there is a gradation in size and all rays are counted (e.g. in Ictaluridae). The count may be expressed in a formula such as iiiI7-8Iiii. The small Roman numerals here represent rudimentary rays, large Roman numerals the unbranched principal rays, and the Arabic numerals the branched principal rays

Dictionary of ichthyology. 2009.

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