4S COOKE : ORIGIN OF THE GENERA OF L. AND F. W. MOLLUSCA. 



On the other hand, the Helicinidce, Hydrocenidce, and Proserpinidcc 

 are equally closely related to Neritina. The Proserpinidcc (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, which in their case is replaced by a 

 number of folds or lamellse 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 freshwater 

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

 perfect sequence of derivatives ; Nerita in the main a purely marine 

 form, with certain species occurring also in brackish water; Neritina in 

 the main freshwater, 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.* 



GASTEROPODA. (2) Inoperculate. 



The origin of these, the bulk of the land fauna, must at present 

 be regarded as an unsolved problem. Bouvier, as we have seen, 

 regards them as derived from the Nudibranchiate Opisthobranchs, the 

 evidence in support of such a view being purely anatomical. No 

 argument can be drawn in this case from the radula, which is very 

 variable in form throughout the Opisthobranchiata, both the great 

 sections of which order include genera possessing radulse of a quasi- 

 Helicidan type, with a formula ccico. 



The first known members of the land Pulmonata {Pupa ?, 

 Hyalinia) are from the Carboniferous of North America. Similar but 

 new forms appear in the Cretaceous, from which time to the present 

 we have an unbroken series. The characteristically modern forms, 

 according to Simroth,'t are Helices with thick shells. According to 

 the same author, Viirina and Hyalinia are ancestral types, which 

 give origin not only to many modern genera with shalls, but to many 

 shell-less genera also, e.g., Tesiacella is derived through Daudebardia 

 from Hyalinia, while from Vitrina came Limax and Amalia. A 

 consideration of the radulae of the genera concerned certainly tends 

 in favour of these views. 



Godwin-Austen, speaking generally, considers! genera of land 

 Pulmonata with strongly developed mantle-lobes and rudimentary 

 shell as more advanced in development than genera in which the 

 shell is large and covers all or nearly all the animal. 



* 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 resumed a purely marine habit of life. 



t SB. Naturf. Gesell. Leipz., 1886-7, PP- 40-48. 



X L. and F. W. Moll, of India, iv., 167. 



