HThe Heteropoda, being warm-water animals should 

 naturally not be expected to be numerously repre- 

 sented in material collected in the northern Atlantic, and 

 the fact that the "Michael Sars" Expedition brought home 

 no less than 12 species must be considered very satis- 

 factory. The genera represented are: Oxygyrus with 

 1 species, Atlanta with 4 (?) species, Carlnaria 1 species, 

 Cardiapoda 1 species, Pterotrachea 4 species, and Firo- 

 loida 1 species. 



Several of these species may, with more or less 

 certainty, be indentified with previously described forms, 

 but some of them may be considered new to science, or 

 they have been so inadequately described that identification 

 is almost impossible. 



Tesch (1906) has rendered a great service to investi- 

 gators by collecting all the species described in literature, 

 with reproductions of the original figures. In dealing with 

 new material, it seems important that full descriptions 

 and illustrations should be given, even of species already 

 described. Only in this way can a knowledge of the 

 comparative anatomy of the Heteropoda, on which to base 

 a natural system be arrived at. 



Atlantidae. 



Heteropoda with a spirally whorled shell large 

 enough to conceal the whole body of the animal and 

 with an operculum. 



The two genera of this family are both represented 

 in the collection. 



Oxygyrus, Benson. 



Species belonging to this genus have a nautiloid 

 shell, the body-whorl being large, horny and translucent, 

 while the narrow spire-whorls may be calcareous. The 

 operculum is triangular, with growth-lines parallel to 

 the bottom side, on both sides curving upwards round 

 an oblique, mytiloid centre at the top. The median 

 plate of the radula has three spines at its free end. 



This genus, so well characterized by its nautiloid, 

 horny shell, is represented by one species. 



O. keraudreni, Rang. 

 PI. I., figs. 1—3. 



Shell (fig. 1 a-b) smooth, consisting of 3 whorls 

 tightly connected and all lying in the same plane. The 

 last whorl is much larger than the others, horny and 

 translucent, while the inner ones are calcified. A broad, 

 horny carina runs from the mouth of the shell along one 

 half of the last whorl, and, suddenly tapering, also along 

 part of the next half. 



As will be seen from the generic description the 

 structure of the operculum is more complicated than was 

 formerly supposed. 



Vayssiere (1904) figures and describes the operculum 

 of O. keraudreni as transversely striated, without a spiral 

 at the top: "l'opercule n'est pas ici spirale comme chez 

 les Atlanta, il est symmetrique et offre seulement de 

 fines stries d'accroissement concentriques." 



And Tesch (1905) in the same way describes it as 

 "dreieckig ohne spiraligen Theil, nur mit parallelen Linien 

 versehen". 



A thorough investigation of the operculum proves, 

 however, that the parallel lines of the lower part curve 

 up at the sides, and form at the top a peculiar oblique 

 centre, very similar to the growth-lines of a mytilus shell 

 (fig. 2 a-b). 



Parts of the radula are drawn in fig. 3 a-b. The 

 three spines of the median plate are very unequal in 

 size, the one in the middle being large and sharply 

 pointed, while the lateral spines are short and blunt, each 

 by a slight incision divided into two minute knots. — 

 The two spines of the intermediate plates are 

 placed one above the other; the lower spine is con- 

 siderably larger than the upper one, and Vayssiere (1904) 

 therefore prefers to characterize the intermediate plates 

 of this species as "unicuspidees". — The lateral plates 

 of the radula are slender and curved, nearly as long as 

 the intermediate plates. 



Of this form three specimens were taken in the 

 Western Atlantic, all at a depth of about 150 m. The 



