488 MOLLUSCA SUB-KINGDOM VI 
Sub-Order B. PTEROPODA. Cuvier.! 
Naked or shell-covered, hermaphrodite, pelagic Mollusca, without distinct head. Eyes 
rudimentary, and foot replaced by two lateral, wing-like 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 transparent 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 Pteropods as an independent class of Mollusca, having equal 
rank with the Gastropods. The researches of Pelseneer, however, have shown that 
they stand in about the same relationship to the Opisthobranchs as Heteropods do to 
the Prosobranchs. They are, in fact, Opisthobranchs which have become pelagic, 
having the foot transformed into a bilobed swimming organ, and with a rudimentary 
head. The structure of the head is very variable. The heart has only one auricle. 
Many shell-covered forms (Limacinidae) develop a horny operculum, but others are 
without it. 
Fossil Pteropods similar to those now hving occur somewhat sparingly in the 
Upper Cretaceous and Tertiary. Argillaceous strata of Pliocene and Oligocene age 
are occasionally highly charged with the remains of Cleodora ; these occur for the 
most part, however, in a compressed and poorly preserved condition. 
Pteropod-like shells are present in the Palaeozoic, appearing as early as the Lower 
Cambrian (Conularia, Tentaculites, Hyolites), where they constitute an important part 
of the fauna. These remains were assigned unhesitatingly to the Pteropods by 
@VArchiac and de Verneuil, Sandberger, Barrande, and numerous other authors. 
Neumayr, however, and more recently Pelseneer, have urged strong objections against 
their union with this group, although they have failed to suggest a more satisfactory 
position for them in the zoological system. Neumayr’s proposal to associate the 
Palaezoic Tentaculites and Styliolas with Tubicolous Worms was rejected by Nicholson 
after a microscopic investigation of their shell-structure. The hypothesis advocated 
by Miller, Fleming, Hall, and more recently by Ihering, according to which Conularias 
are Cephalopods akin to Orthoceratites, is utterly without any credible foundation. 
1 Literature (see also preceding bibliographies) : 
Sandberger, G., Die Flossenfiisser oder Pteropoda (Neues Jahrb. fiir Mineral., pp. 8-25), 1847. 
Barrande, J., Pugiunculus, ein fossiles Pteropoden-Geschlecht (Neues Jahrb. fiir Mineral., pp. 554- 
558), 1847 ; Systeme Silurien du centre de la Bohéme, vol. III., Ptéropodes, 1867. 
Salter, J. W., Memoirs of the Geological Survey of Great Britain, vols. II., IIT., 1848, 1866. 
Seguenza, G., Paleontologia malacologica dei terreni terziarii del distretto di Messina. Pteropodi ed 
Eteropodi (Mem. Soc. Ital. Sci. Nat. Milano, vol. IT.), 1867. 
Karpinsky, A., Die fossilen Pteropoden am Ost-Abhang des Ural. (Mem. Acad. St. Petersburg, ser. 
7, vol. XXXII., pp. 1-20), 1884. 
Dolifus, G., and Ramond, G., Liste des Ptéropodes du terrain tertiare parisien (Mem. Soc. Malacol. 
de Belgique, vol. XX.), 1885. 
Walcott, C. D., Contribution to Studies on the Cambrian Faunas of North America (Bull. U.S, Geol. 
Survey, vol. [V., No. 30, pp. 125-146), 1886 ; 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 ; 
Bull. Soe. Belge de Geol. Palaeont. et Hydrol., vol. III. p. 124), 1889. 
Blanckenhorn, M., Pteropodenreste aus der oberen Kreide Nord-Syriens und aus dem hesseschen 
Oligoeiin (Zeitschr. deutsch. geol. Gesellsch., vol. XLI. pp. 595-602), 1889. 
Novak, O., Revision der palaeozoischen Hyolithiden Bohmens (Abhandl. bohm, Gesellsch. Wissensch. [7] 
Bd. IV.), 1891. 
Holm, G., Sveriges Kambrisk-Siluriska Hyolithidae och Conularidae (Afhandl. Sver. geol, Under- 
sdkning, Ser. C, No. 112), 1893. ° ; 
