416 MANUAL OF ZOOLOGY. 



the Holostomata are more abundant in the Palaeozoic period, 

 the Siphonostomata abounding more in the Secondary and 

 Tertiary rocks, but not attaining their maximum till the pres- 

 ent day. The place of the carnivorous Siphonostomata in the 

 Palaeozoic seas appears to have been filled by the Tetrabran- 

 chiate Cephalopods. The Branchiate Gasteropods of fresh 

 water are chiefly represented as fossils by the genera Melania, 

 Paludina, Valvata, and Ampullaria. 



The Heteropoda are likewise of very ancient origin, having 

 commenced their existence in the lowest Silurian deposits. 

 The genera Bellerophon, Porcellia, Cyrtolites, and Maclurea, 

 are almost exclusively Palaeozoic ; Bellerophina is found in the 

 Gault (Secondary), and Carinaria has been detected in the 

 Tertiaries. 



The Pulmonate Gasteropoda, as was to be anticipated, are 

 not found abundantly as fossils, occurring chiefly in lacustrine 

 and estuarine deposits, in which the genera Limncea, Physa, 

 Ancylus, &c., are amongst those most commonly represented. 

 These, however, are entirely Mesozoic and Kainozoic. In the 

 Palaeozoic period the sole known representatives of the Pul- 

 monifera are the Pupa vettista, Pupa vermilionensis, Dawsonella 

 Meeki, and Zonites priscus of the Carboniferous rocks. 



CHAPTER XLVII. 

 PTEROPODA. 



CLASS III. PTEROPODA. The Pteropoda are denned by being 

 free and pelagic, swimming by means of two wing-like appendages 

 (epipodia), developed from each side of the anterior extremity of 

 the body. The flexure of* the intestine is neural. 



As to the position of the Pteropoda in the Molluscan scale, 

 they must be looked upon as inferior in organisation to any of 

 the Gasteropoda, of which class they are often regarded as the 

 lowest division. They permanently represent, from certain 

 aspects, the transient larval stage of the Sea-snails. 



The Pteropods are all of small size, and are found swimming 

 at the surface of the open ocean, often in enormous numbers. 

 Locomotion is effected by two wing-like fins, developed from 

 the sides of the head, and composed of the greatly-developed 

 "epipodia." The true "foot" is rudimentary and rarely dis- 

 tinct, but the " metapodium " is sometimes provided with an 



