488 MOLLUSCA SUB-KINGDOM vi 



Sub- Order B. PTEROPODA. Cuvier. 1 



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 slid! 

 (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 living 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 

 d'Archiac and de Verneuil, Sandberger, Barrande, and numerous other authors. 

 Xeumayr, 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. fii'r Mineral., pp. 8-25), 1847. 

 Barrande, J., Pugiunculus, ein fossiles Pteropoden-Geschlecht (Neues Jahrb. f'iir Mineral., pp. 554- 



558), 1847 ; Systeme Silurien du centre de la Boheme, vol. III., Pteropodes, 1867. 

 Salter, J. W., Memoirs of the Geological Survey of Great Britain, vols. II., III., 1848, 1866. 

 Seguenza, G., Paleontologia malacologica dei terreni terziarii del distretto di Messina. Pteropodi ed 



Eteropodi (Mem. Soc. Ital. Sci. Nat. Milano, vol. II.), 1867. 

 Karpinsky, A., Die fossilen Pteropoden am Ost-Abhang des Ural. (Mem. Acad. St. Petersburg, ser. 



7, vol. XXXII., pp. 1-20), 1884. 

 Dollfus, G., and llamond, G., Liste des Pteropodes du terrain tertiare parisien (Mem. Soc. Malacol. 



de Belgique, vol. XX.), 1885. 

 Walcott, G. I)., Contribution to Studies on the Cambrian Faunas of North America (Bull. U.S. Geol. 



Survey, vol. IV., 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. Soc. Beige de Geol. Palaeont. et Hydrol., vol. III. p. 124), 1889. 

 Blanckenhorn, M., Pteropoclenreste aus der oberen Kreide Nord-Syriens und aus dem hesseschen 



Oligocan (Zeitschr. deutsch. geol. Gesellsch., vol. XLI. pp. 593-602), 1889. 

 Novdk, 0., Revision der palaeozoischen Hyolithiden Bohmens(Abhandl. bohm. Gesellsch. Wissensch. [7] 



Bd. IV.), 1891. 

 Holm, G., Sveriges Kambrisk-Siluriska Hyolithidae och Conularidae (Afhaudl. Sver. geol. Under- 



sokuing, Ser. C, No. 112), 1893. 



