TROCHUS. 293 



haps are still) called in some parts of the north of Italy 

 " trottola," by the fishermen at Spezzia " narnai," and 

 by the French " culs-de-lampe." Adanson's species of 

 Trochus belong to Littorina. Dr. Leach's posthumous 

 ' Synopsis of the Mollusca of Great Britain ' contains 

 an extremely inaccurate account of the anatomical 

 structure of the animal. The following are extracts : 

 " The eggs (ova) are pedunculated ; the peduncle is 

 situated at the sides of the tentacles of the young 

 animal : " we are also told that Trochus has " four 

 tentacles." Surely the publication of such a work was 

 not "an act of justice" to the memory of this once 

 celebrated zoologist. In the 'Zoologia Adriatica' of 

 Olivi will be found some curious lucubrations as to the 

 cause of the internal lustre, resembling silver or mother 

 of pearl, which decorates the shells of this genus. 

 Finding that the shell of Trochus was composed of 

 different layers, he at first supposed that the iridescence 

 could only be the effect of light reflected or refracted at 

 different angles from the distinct surfaces which resulted 

 from the relative superposition of these layers. In con- 

 sequence, however, of the experiments made by Herissant 

 with respect to the heterogeneous nature of shell-matter, 

 and of Bouvier having detected by analysis a consider- 

 able proportion of magnesium in Corallina officinalis, 

 Olivi hazarded another conjecture, viz. that the irides- 

 cence might arise from the admixture of some other 

 mineral with carbonate of lime, such as is seen in mica 

 schist. Nacre composes the inner layer of every 

 species, and the entire substance of some; and Car- 

 penter was able to distinguish in this nacreous composi- 

 tion the same minutely cellular arrangement which he 

 had described as presenting itself so distinctly in Haliotis. 

 The genus comprises a multitude of species, recent and 



