82 PALAEONTOLOGY. 



the Atlantidw, which can retire into their shells and close them 

 with an operculum. The known fossil forms belong chiefly to 

 the latter division. Both animal and shell are symmetrical, 

 or nearly so ; the nucleus of the shell is minute and dextrally 

 spiral. 



The soft parts of Atlanta are divisible into a "somatal" 

 and a "pallial" region. In fig. 24, the former or chief fleshy 

 part of the body is out of the shell; the pallial or visceral 

 part is in the shell, which latter is an appendage to it. The 

 " soma" is divided into the cephalic A, pedial B, and the lid- 

 bearing tail or operculigerous lobes e,f. The head, or cephalic 

 lobe, includes the mouth-mass a, the tentacles h, and the eyes 

 /; the foot is divided into the "fin" B, and the "disc" d; the 

 tail includes the "leaf" e, and the "lid" or opercule/, with its 

 surface of attachment, / is the gullet, m the crop, n the 

 stomach, o the intestine, jo the liver, a the kidney, s the heart, 

 h the branchial chamber, i the gill, u v the chief ganglions of 

 the nervous system. The shell of the Atlanta, besides the 

 beauty and symmetry of its shape, purity of colour, and 

 delicacy of texture, is remarkable for combining two conditions 

 of shell-tissue; retaining a large proportion of the mouth, or 

 last-formed part, in a soft flexible quasi-cartilaginous state, 

 the rest of the shell being vitreous. Only the body-part, 

 therefore, could be expected to become fossilized, and this 

 circumstance should be borne in mind while comparing those 

 fossil univalves, which in their symmetry resemble the 

 Nautilus, but are unfurnished with air-chambers. Such most 

 probably belong to the Nucleobranchiata, and especially to 

 that division typified by the Atlanta. The genus Porcellia, 

 characteristic of the carboniferous age, has a discoidal shell, 

 with a spiral nucleus projecting, as in Atlanta, from the right 

 side; the whorls are exposed, and marked with a narrow band 

 along the back, ending in a deep slit (fig. 26, 6). Another 

 genus (Bellerophon) resembles the recent Oxygyrus in its more 



