814 THE PALEONTOLOGY OF MINNESOTA. 



[Introduction. 



"odontophore." The principal portion of this is a chitinous band called the 

 "radula" or "lingual ribbon." This is beset with minute, regularly arranged teeth, 

 and, as it is supported on a cartilaginous cushion, which can be given a rotary 

 motion by a special set of muscles, the ribbon is made to act as a file or saw. The 

 arrangement of the teeth in the radula, taking the whole class into consideration, is 

 subject to considerable variation; yet, within narrower limits, their form and 

 disposition are so constant that they afford one of the most reliable aids to classifi- 

 cation. As a rule they are disposed in a medium series, flanked by two or more 

 lateral rows. 



Two principal modes of respiration pertain to the Gastropoda,ihe first by means 

 of gills variously constructed and adapted to breathe air dissolved in water, and the 

 second by means of a pulmonary chamber which is adapted for aerial respiration. 



The alimentary canal, nervous and vascular systems, and some of the senses 

 (seeing, feeling and hearing) are well developed. The sexes may be separate or 

 united in one individual. Generally the young are developed in eggs laid in horny 

 capsules or in the form of a string or band. With very few exceptions the young 

 when first hatched are provided with a shell, which in some cases may subsequently 

 entirely disappear through resorption. Very commonly the embryonic shell forms 

 a nucleus at the apex of the fully-grown shell which is often very different from 

 the following portions. 



Most of the Gastropoda are water animals and a large proportion of them are 

 inhabitants of the sea. Only the Pulmonata and certain groups of the Prosobrancliiata 

 are terrestrial in habit or live in fresh waters. The pteropods and heteropods of the 

 present day are oceanic in habit and are found swimming in the open sea, near the 

 surface and far from land; but the majority of the marine prosobranchs and the 

 opisthobranchs live in shallow seas, many of them between tide-marks. In depths 

 exceeding five hundred fathoms the number of Gastropoda is greatly reduced; 

 still a few forms are found to inhabit depths of two or three thousand fathoms, or 

 even more. 



The shell, with which, for manifest reasons, the paleontologist is chiefly concerned, 

 is a secretion of the mantle. It is wanting in the adult stages of the nudibranchs, 

 while in other cases, notably the slugs, it is very minute and hidden in the mantle. 

 In its chemical composition carbonate of lime, either in the form of calcite or 

 aragonite, constitutes 95 per cent, or more of the whole. The inner layer is often 

 nacreous (PleurotomariidiF), but as a rule the whole shell, aside from the horny 

 epidermis, consists of an apparently homogeneous porcellaneous mass, which when 

 carefully examined is seen to be made up of three layers composed of parallel 

 ', those in the outer and inner layers having the same direction, i. e. perpen- 



