CLAss IV GASTROPODA 569 



Suborder C. PTEROPODA Cuvier.i 



Nahed or shell-covered, hermaphroditic, pelagic Mollusca, without distinct head. Eyes 

 riidimentary, and foot modified so as to forra tvjo lateral, wing-like fins, situated on the 

 anterior end of the hody. The gills are placed hehind the heart. 



The body of these free-swimming Mollusca is sometinies elongated, sometimes coiled 

 posterioiiy in a si>iral. In some instances it is covered by a thin transparent sliell 

 {Thecosomata), biit oftener it is naked (Gymnosomata). The creatures associate in vast 

 swarms in the open sea, and rise to the surface toward nightfall. Their shells often 

 accumulate in prodigioiis quantities on the sea-bottom, forming calcareous deposits of 

 considerable magnitude. 



C 11 vier recognised the Pteropods as an independent class of Mollusca, having equal 

 rank with the Gastropods. Modern researches, however, have approximated theni 

 niore closely to the latter througli the swimming Opisthobranchs. If \ve regard it as 

 probable that invertebrate life began in the sea, it almost certainly follows that 

 Pteropods are among the earliest Mollusca. Also, granting that the conditions of 

 their existence have undergone no appreciable change since the ocean becanie capable 

 of sustaining such pelagic life, there is no obvious reason why the members of the 

 group should have since experienced any radical modification, 



The earlier paleontologists, d'Archiac, de Verneuil, Sandberger, Barrande and 

 others, recognised the true relations of the Paleozoic Pteropods, though uniting with 

 them some forms of similar appearance, which probably are not of niolluscan nature, 

 such as Conularia, and perhaps Tentaculites. 



Neumayer and Pelseneer, led by preconceived theories, have objected to the union 

 of Paleozoic forms like Hyolithes with the Pteropods, though proposing no satisfactory 

 alternative ; and by a curious reversal of paleontologic succession, have wished to 

 derive the Pteropoda from the more modern Opisthobranchs. Since the anatomy of 

 the Cambrian forms seemed inaccessible, the uncertainty bade fair to remain permanent, 

 when, by the discovery of the wonderful Middle Cambrian deposits of the Canadian 

 Rocky Mountains, among the otlier fossils showing traces of the soft parte, were found 

 several specimens of Hyolithes carinatus Matthew, with distinct and clear impressions 

 of the pteropodia. These, judging from the sharpness of their anterior margins, seeni 

 to have had there some kind of a chitinous support, perhaps like the chitinous rods 

 supporting the gill-lamellae of some Nuculidae.^ This wholly unexpected confirmation 

 of the earlier view as to the relations of these fossils, falls in with the views generally 

 held by malacologists as to the derivation of the swimming Opisthobranchs from the 



^ Literature (see also preceding bibliograpliies) : Sandherger, O., Die Flossenfüsser oder Ptero- 

 poda. Neues Jahrb. für Mineral., pp. 8-25, 1847. — Barrande, J., Piigiunculus, ein fossiles 

 Pteropoden-Geschlecht. Neues Jahrb. für Mineral., pp. 554-558, 1847.— Systeme Sibirien du centre 

 de la Bolieme, vol. iii. Pteropodes, 1867. — Salter, J. W., Menioirs of the Geological Survey of 

 Great Britain, vols. ii., iii., 1848, ISQQ.—Seguenza, G., Paleontologia raalacologica dei terreni terziarii 

 del distretto di Messina. Pteropodi ed Eteropodi. Mem. Soc. Ital, Sei. Nat. Milano, vol. ii., 1867. 

 — Karpinsky, A., Die fossilen Pteropoden am Ost- Abhang des Ural. Mem. Acad. St. Petersbourg, 

 ser. 7, vol. xxxii. pp. 1-20, 18Si.—Doll/us, Q. and Ramond, G., Liste des Pteropodes du terraiu 

 tertiaire parisien. Mem. Soc. Malacol. de Belgique, vol. xx., \9>8b. — Walcott, C. D., Contribution 

 to Studies on tlie Cambrian Faunas of North America. Bull. U. S. Geolog. Survey, vol. iv. No. 30, 

 pp. 125-146, 1866.— The Fauna of the Lower Cambrian or Olenellus Zone. Tenth Ann. Rept. 

 U.S. Geol. Survey, 18^0.— Pelseneer, P., Report on the Pteropoda. Report Challenger Expedition, 

 Zoology, vol. xxiii., 1888.— /«ie?/i, Bull. Soc. Beige de Geol. Palaeont. et Hydrol., vol. iii., 1889.— 

 Blanckenhorn, M., Pteropodenreste aus der oberen Kreide Nord-Syriens und aus dem hessischen 

 Oligocän. Zeitschr. Deutsch. Geol. Ges., vol. xli., 1889.— iVb^'(i^^ 0., Revision der paläozoischen 

 Hyolithiden Böhmens. Abhandl. Böhm. Ges. Wiss. [7] vol. iv., 1891.— //oZm, G., Sveriges Kam- 

 brisk-Siluriska Hyolithidae och Conularidae. Afhandl. Sver. geol. Undersök., Ser. C, No. 112, 

 1893.— AS^^a^er, /., Monograph of British Conulariae. Palaeont. Soc, 1^01 . — Walcott, C. D., Cam- 

 brian Geology and Paleontology. Smiths. Mise. Coli., 1912, vol. Ivii., No. 5. 



- A figure of this fossil is given by Walcott in Smithson Mise. Coli., 1912, vol. Ivii., No. 5. 



