In a seasonal series of plankton samples, collected by Professor D' ANCONA (1938) in the marsh of Fucecchio (near Florence), among the species of Euchlanis a new form has been found . After these samples were taken, a land reclamation was undertaken (the Fucecchio marsh covers a surface of about 20 sq. km) through the deepening of a drain canal . Now the rainfall drain is very fast and the amount of stagnant waters is strongly reduced . The environment appears very different from the previous situation, since the place has been changed into a network of canals . A new collection of plankton samples carried out recently has shown that the species of the genus Euchlanis have disappeared with the exception of the ubiquitous E. dilatata* .
The Euchlanis species of Fucecchio have in common a remarkable peculiarity of the mastax : the functional teeth of the uncus are all connected by a thin chitinous sheet from their basis to about the beginning of their terminal enlargement, so that the uncus consists of two roughly square laminae where the teeth appear as cord-like thickenings, free only at the knotty end. Furthermore the last dorsal teeth often exhibit a double branching . *) A full account of the change of the plankton facies will be given in an other paper .