I ORIGIN OF LAND OPERCULATES 21 



The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, 

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

 have very close relations, particularly in the length and forma- 

 tion of the radula, or lingual ribbon, with the Littorinidae. 



Fig. 17. — Two rows of the radula of Cyclophorus sp., India, x 40. 



On the other hand, the Helicinidae, Hydrocenidae, and Pro- 

 serpinidae are equally closely related to Neritina. The Pro- 

 serpinidae (restricted to the Greater Antilles, Central America 

 and Venezuela) may perhaps be regarded as the ultimate term 

 of the series. The}^ have lost the characteristic operculum, 

 which 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 



A 



Fig. 18. — A, Nentina reticularis Sowb., Calcutta (brackish water); B, Helicina 

 neritella Lam., Jamaica (land) ; C, Proserpina {Geres) eolina Duel., Central 

 America (land). 



purely marine form, with certain species occurring also in 

 brackish 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 Proserpina^ an 

 aberrant derivative which has lost the operculum.^ 



1 One step even further (or perhaps it should be termed a branch derivative) 

 is seen in the genus Smaragdia, which is probably a Neritina which has re- 

 sumed a purely marine habit of life. 



