ORIGIN OF LAND OPERCULATES 



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Tlie Cycloplioridae, Cyclostomatidae, and Aciculidae, Avliicli, 

 as contrasted with the other land operculates, form one group, 

 have very close relations, particularly in the lengtli and formation 

 of tlie raduLi, or lingual ribbon, with the Littorinidae. 



Frfi. 17. — Two rows of the radula of Cijdophorus sp., India, x 40, 



On the other hand, the Helicinidae, Hydrocenidae, and 

 Proserpinidae are equally closely related to Neritina. The 

 Proserpinidae (restricted to the Greater Antilles, Central America 

 and Venezuela) may perhaps be regarded as the ultimate term 

 of the series. They have lost the characteristic operculum, 

 wliicli in their case is replaced by a number of folds or lamellae 

 in the interior of the shell. It has already been noticed how 

 one group of Neritina {Neritodryas) occurs normally out of the 

 water. This group furnishes a link between the fresh-water and 

 land forms. It is interesting to notice that here we have the 

 most perfect sequence of derivatives ; Nerita in the main a 



Fkj. 18. — A, Neritina reticularis Sowb., Calcutta (brackish water) ; B, Helicina 

 neritella Lam., Jamaica (land) ; C, Proser})ina [Ceres) eolina Duel., Central America 

 (hmd). 



piirely marine form, with certain species occurring also in 

 lirackish water; Neritina in the main fresh -water, but some 

 species occurring on the muddy shore, others on dry land : 

 Helicina the developed land form ; and finally Proseiyina, an 

 aberrant derivative which has lost the- operculum.^ 



' One step even further (or perhaps it slioukl be termed a branch derivative) is 

 seen in the genus Smaraydia, which is probably a Neritina whicli lias resumed a 

 purely marine habit of life. 



