230 SYNOPTIC COLLECTION. 



primitive lungs in the form of a pulmonary sac or cavity. 

 As a proof that the Pulmonifera are more specialized 

 than the groups already described, it may be stated that 

 many Pulmonates fill their lung sac with fresh water in 

 the younger stages and only later in life dispense with 

 the water. There are objections to the use of the term 

 Pulmonifera, since some of the air-breathing Gastropods 

 (Cyclostoma, for instance; see No. 461) are so different 

 in structure from the typical Pulmonifera that they can- 

 not be placed in this group but are specialized air-breath- 

 ing members of more generalized groups. 



As a rule the shells of the Pulmonifera do not attain 

 that brilliancy of coloring and that extreme ornamenta- 

 tion which may be observed in their marine ancestors. 



Among the fresh- water and land Gastropods there are 

 no forms possessing the straight cone-like shell, but the 

 shallow cup of Patella and the spiral of Crepidula are 

 represented by Ancylus and Gundlachia. These Gastro- 

 pods have habits similar to those of their marine relatives 

 and the structure is similar. We have not found a spiral 

 so loosely coiled as Scala ; the majority represent the 

 group of tightly coiled plain spirals, the chief difference 

 between the genera being in the plane of the coiling. 



Ampularia globosa Swains. (No. 497), is a simple spiral 

 with a small umbilicus, holding a similar place among 

 fresh-water univalves that Lunatia holds among the ma- 

 rine. The glossy, horny layer of this genus is striking, 

 reminding one of this finely developed layer on fresh- 

 water bivalves. The operculum of the young and the 

 adult shell is not spiral, but the additions are made con- 

 centrically (see No. 497). 



In this genus the left gill is vestigial, the mantle cavity 

 having a large pulmonary sac on each side. These mol- 

 luscs are amphibious, being able to live a long time out 

 of water. 



Another fresh-water form is the pond snail, Limnaeus 

 stagnalis O. F. Mull. (No. 498) . It comes to the sur- 



