MOLLUSCS. 355 



insj the South Atlantic Ocean were vast mimbers of Atlantce and numerous Carinarice. 

 They are crepuscular animals like the pteropocis, and are furnished with hyaline shells 

 of the greatest delicacy and beauty. The Atlanta, with an elegant, glassy, spiral, 

 carinated shell, globose in one sjjecies and flattened in another, is quite a sprightly 

 little mollusc, probing every object within its i-each by means of its elongated trunk, 

 twisting its body about, and swimming in every direction by the lateral movements of 

 its vertical dilated foot. I have frequently seen them descend to the bottom of the 

 glass vessel in which they were kept, iix themselves there in the manner of a Icecli by 

 their sucking disc, and carefully examine the nature of their prison by protruding the 

 front jjortion of their foot in every direction. Almost all pelagic molluscs usually 

 swim on their backs in a reversed position, and although I have seen them commonly 

 swim in this way after capture, they frequently progress feebly with the shell upper- 

 most. When fresh and just taken, I have seen both Atlanta and Carinaria swim 

 with their bodies in every position ; 'on their sides, on their backs, and with the foot 

 downwards. The Carinarice are swift and rapid in their movements, and dart for- 



A(P. ^-^ '•. , 



Fio. 465. — Pterolrachea scutata; a, shield; 6, proboscis; c, mouth; d, float; e, sucker; g, cloacal sac; A, gills; 

 t, genital gland; I, alimentary canal; m, eye. 



wards by a continuous effort, moving their foot and caudal ap]iendage from side to 

 side as a powerful natatory organ ; they do not progress by sudden jerks like Atlanta. 



Three well-marked families of Heteropoda are recognized by naturalists. In the 

 Atlantid^ the visceral hump is well developed, and is enclosed in a spiral, glassy 

 shell, large enough to contain the animal when retracted. The shell starts in a dex- 

 tral spiral, but soon becomes flattened and bilaterally symmetrical. Its edge is pro- 

 vided with a sharp keel, and the aperture is closed on the retraction of the animal by 

 a thin ovate oi)erculum. But two genera are known, Atlanta and O.vi/ffi/rns, contain- 

 ing a total of about twenty-five species, all from the warmer seas. The fossil genus 

 lielkrophon possibly belongs here. 



In the CARiNARTD.i: the visceral hump and the shell have undergone considerable 

 reduction in size, as shown by our figure. The shell is hyaline, and from beneath its 

 margin project the gills. The middle lobe of the foot is alone pi-ominent, and extends 

 from the lower surface of the body like a vertical fin. The tentacles arc well devel- 

 oped. The tvv'o genera Carinaria and Cardiapoda contain a baker's dozen of species. 



In the Pterotracheid^ the reduction of the visceral hump is carried to its 

 fullest extent. The animal forms a long, cylindrical body, relieved only by the ven- 

 tral foot and the small and inconspicuous gills, which are unprotected in some forms, 



