The largest

Every group is ranked on its own, on one measurement, and the measurement is named on every row.

That is not caution. A snake and a turtle are measured with different rulers: a snake snout to vent, leaving its tail out of the figure entirely, and a turtle across the shell. Put them in one column and the ranking would be about the ruler rather than the animal.

Ranked on snout-to-vent length, which 15,251 species carry. 6,321 have no figure recorded at all and cannot appear here. No published body length exists for them in any source this encyclopedia reads.