The same mistake

John Edw. Gray, « On the « Freshwater Carriers », or Thelidonus of Mr. Swainson », Journal of Natural History, Series 1, vol. 5. Issue 31, 1840, p. 257-258.

Mr. Swaison in his Malacology, a recent volume on ‘Shells and Shell-fish, » has established a genus, under the name Thelidomus, upon two bodies which he received with some Planorbes from Brazil. These «  two Helix-looking shells, » as he calls them, he considers as «  absolute counterparts of «  the carrier Trochi, and states that they « fill the same situation among the Rotellinae which Onustus ( the carrier Trochus) does among Trochine. »He also represents them as forming a passage from these animals to the Helices, and observes that the «  discovery of the extraordinary shell will probably induce naturalists to a more accurate examination of the fossil turbinated univalves than they have received ; for it is clear, that, although Thelidomus opens the path to the Helicidae, there must be several other forms between the two, either extinct or undiscovered, » p. 228.-The bodies which he hrre describes, and the « singular discovery » which he represents as throwing an «  entirely new light on this interesting question », are «  actually composed of little stones and grains of sand only, agglutinated together, » p.227 and 353, and are the cases of a caddis worm which is very common in Brazil and the United States. It differs from the cases of the European kinds in being spiral ; but it agrees with them in being pervious at each end, though the hole at the smaller end appears to have escaped Mr Swainson’s observation. The North American cases have been described by Mr. Lea, under the name of Valvata arenifera. It might have been supposed that the ridicule which Mr. Lea has incurred for this error, would have prevented any other naturalist from falling into the same mistake. Indeed it is difficult to understand how any one who considers the formation and structure of shells, should not have reflected on the peculiarity of the formation og these bodies before he described them, and proceeded to build up such important reasonings upon their supposed place in his system.