38 INVERTEBRATE ANIMALS. 



loith other Gasteropoda, t]ie Land-snails and Slugs possess a similar 

 odontopliore or elastic strap, set icith sharp teeth; transformations less 

 distinct; a priirumj io.e\a:&\ flexm-e of the intestine ; and hreatlie air brj 

 means of a pulmonary chamber formed by an inflexion of tJie mantle. 



C^rtt^ Cabk Case. 



Subclass GASTEROPODA PULMONIFERA. 



TTveujacov, lungs ; <^spM, I carry. Not more tbau about 

 balf-a-dozen forms bave been found in palaeozoic rocks. 



Group 109.— Order OPERCULATA. operculum, a lid. 

 Estimated number of species: recent, 1,000; British, 3; 

 fossil, 50 ; Tertiary. The operculate Pulmonifera attain 

 their highest development in the Polynesian Islands, 

 but are found in both hemispheres. The shells exhibit 

 remarkable varieties in form and sculpture : some are 

 cylindrical and glassy, others discoid ; many are beauti- 

 fully keeled, as may be seen in the delicately fringed 

 whorls of Trochatella constellata. The drawer is 

 crowded, but contains examples of nearly all the generic 

 and sub-generic forms. 



H Certain sections in this Order are supposed to 

 have closer affinity with some of the con- 

 stituents of the Sub -class Gasteropoda 

 Branchifera. 



Order INOPERCULATA. in, without ; oper- 

 culujii, a lid. 



Group 110.— Sub-order THALASSOPHILA. SaXacrcra, 

 the sea ; <plxoc, a friend. Estimated number of species : 

 recent, 100 ; British, ; fossil, 3. Tertiary. 



H Examples of Siphonaria, Amphibola, &c. The 

 shells of Siphonaria resemble those of the 



