82 LECTURES ON MOLLUSCA. 



nella: and the East Indian Camptoceras has the whirls separated like 

 Vermetus. Fossils of this tribe, as of Limncea and Planorbis, are 

 found as old as the Wealden oolitic rocks. 



The limpet-like shell of Ancylus is as different from PJiysa as Brode- 

 ripia from Trochus, or Testacellus from Glandina. Nevertheless the 

 animal is even more closely allied. The shell is sinistral, (the point 

 being turned to the right,) and entirely covers the animal ; which has 

 much less attachment to it than the Limpets, and can move its long 

 neck freely under its large umbrella. Velletia is a dextral shell, with 

 the apex turned to the left, and a somewhat different arrangement of 

 teeth. Both forms are found fossil in Eocene strata. The curious 

 little New Zealand Latia has a deck across one end, like the Slipper- 

 limpets. Lastly, the Cuban GundlacMa has the knobby apex pro- 

 duced, and the deck broad, so as to resemble some of the small-spired 

 Neritince, but without operculum. All these curious freshwater Lim- 

 pet-snails crawl on stones or plants, generally in clear water. 



TRIBE III. THALASSOPHILA. (Marine-snails . ) 



These curious creatures are always found close to the sea. The ani- 

 mals greatly resemble Auricula, and have the normal dentition of 

 Helix. The inside of the breathing chamber is wrinkled, so that it 

 would appear that neither air nor water would come amiss. The 

 cavity is however closed as in the true snails, and wet sea air is prob- 

 ably most congenial to them. The small tentacles are flattened out 

 into a disk round the head. 



Family AMPHIBOLIES. (Periivinkle-snails.) 



These creatures have shells somewhat like a Natica, with the outer 

 lip somewhat notched, as though for an air passage. They are eaten 

 in New Zealand like Periwinkles, and differ from all other true Pul- 

 monates in having a thin, horny, sub-spiral operculum. There is 

 only one genus known, Amphibola, from the Australian seas. 



Family SIPHONARIAD^E. (Sea Limpet-snails.} 



The Siphonarias have solid, conical shells, often overgrown with 

 sea-weeds and nullipores. They are known from Limpets by their 

 irregularity of form, caused by a groove which interrupts the muscle 

 of attachment on the right side; not traversing it, as in Gadinia. 

 They are found on almost all tropical shores. There is a large man- 

 tle-flap covering up the breathing hole. The tentacles are entirely 

 flattened down into a veil ; and the animal has a much plainer appear- 

 ance than the ordinary Limpets. The individuals in many species vary 

 more, in shape and sculpture, even than in their water-breathing 

 neighbors. These creatures are to the Amphibolce what the Ancylus is 

 to the Planorbis. 



SUB-CLASS. OPISTHOBKANCHIATA. 



The next division of the crawling mollusks consists of creatures 

 which are generally destitute of shells, or simply have them as a pro- 



