CLASS IV GASTROPODA 569 



Suborder C. PTEROPODA Cuvier.^ 



Naked or shell-covered, herviapliroditic, i)elagic Molhisca, ivithout distinct head. Eijes 

 rudimentary, and foot modified so as to form tvjo lateral, wing-lihe fins, situated on the 

 anterior end of the body. The gills are placed behind the heart. 



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

 posteriorly in a spiral. In some instances it is covered by a thin transj^arent shell 

 (Thecosomata), but 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 prodigious quantities on the sea-bottom, forming calcareous deposits of 

 considerable magnitude. 



Cuvier recognised the Pterojiods as an independent class of Mollusca, having equal 

 rank with the Gastropods. Modern researches, however, have apjjroximated them 

 more closely to the latter through the swimming Opisthobranchs. If we regard it as 

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

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

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

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

 grouji 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 molluscan nature, 

 such as Conularia, and perhaps Tentacnlites. 



Neumayer and Pelseiieer, led by preconceived theories, have objected to the imion 

 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 PterojDoda 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 other fossils showing traces of the soft parts, 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, seem 

 to have had there some kind of a chitinous supjDort, perhaj)s 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 OiJisthobranchs from the 



^ Literature (see also preceding bibliographies) : Sandberger, G., Die Flossenflisser oder Ptero- 

 poda. Neues Jalirb. liir Mineral., pp. 8-25, 1847. — Barrande, J., Pugiunculus, ein fossiles 

 Pteropoden-Gesclilecht. Neues Jahrb. fiir Mineral., pp. 554-558, 1847. — Systenie Silurien du centre 

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

 Great Britain, vols, ii., iii., 1848, 1866. — Se.guenza, G., Paleontologia malacologica dei terreni terziarii 

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

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

 ser. 7, vol. xxxii. pp. 1-20, 1884. — Doll/us, G. and Ramond, G., Liste des Pteropodes du terrain 

 tertiaire parisien. Mem. Soc. Malacol. de Belgique, vol. xx., 1885. — TfWco«, C. D., Contribution 

 to Studies on the 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, 1890. — Pelseneer, P., Report on the Pteropoda. Report Challenger Expedition, 

 Zoology, vol. xxiii., 1888.— /c?e»i, Bull. Soc. Beige de Geol. Palaeont. et HydroL, vol. iii., 1889.— 

 Blanckenhdrn, M., Pteropodenreste aus der oberen Kreide Nord-Syrieus uud aus dem hessischen 

 Oligocan. Zeitschr. Deutsch. Geol. Ges., vol. xli., 1889. — NovCik, 0., Revision der palaozoischen 

 Hyolithiden Bohmens. Abhandl. Bohm. Ges. Wiss. [7] vol. iv., 1891.— i/oZm, G., Sveriges Kam- 

 brislv-Siluriska Hyolitlddae och Conularidae. Afhandl. Sver. geol. Underscik., Ser. C, No. 112, 

 l^^^.— Slater, I., Monograph of British Conulariae. Palaeont. Soc, 1%Q1 . — Walcott, C. D., Cam- 

 brian Geology and Paleontology. Smiths. Misc. Coll., 1912, vol. Ivii., No. 5. 



"A figure of this fossil is given by Walcott in Smithson Misc. Coll., 1912, vol. Ivii., No. 5. 



