TYPE MOLLUSOA. 307 



single auricle only, except in Cyprcea, wliere the rudiment of a 

 second occurs, and throughout the group but a single nephrid- 

 ium is present. There is never more than a single gill, which 

 is usually more or less united to the mantle-wall and bears 

 lateral branches only upon one side. 



The foot is in some cases flat and broad, as in the Dioto- 

 cardia, and in such cases may possess the parallel pedal nerve- 

 cords with transverse commissures as in Cyprcea and Palvdina, 

 but usually it becomes more or less differentiated, a propodi- 

 um being in many cases well defined {Strombus, Natica), while 

 a chitinous or calcareous operculum is usually carried by the 

 metapodium, and the pedal nerve-cords are very much re- 

 duced or wanting, the pedal ganglia being on the other hand 

 more highly developed than in the Diotocardia. The epipo- 

 dium is usually entirely wanting, and when present is but 

 slightly developed, reaching its fullest development as a con- 

 tinuous fold upon the sides of the foot only in lanthina. In 

 JPcUvdina it is represented by two anteriorly- situated tentacle- 

 like lobes, and in Calyptrcea by a semicircular fold on each 

 side of the neck region. 



The Monotocardia are further distinguished by the fre- 

 quent occurrence of a well-developed siphon and a more or 

 less developed siphon groove at the margin of the shell, and 

 furthermore a well-developed penis is usually present. 



The anterior portion of the digestive tract is in many forms 

 capable of being protruded as a proboscis. The arrangement 

 of the teeth of the radula varies considerably in different 

 forms, but the rhipidoglossate arrangement is not represented. 

 In one group, including the genera Cyprcea, Natica (Fig. 137, 

 A), Littorina (the periwinkles), Calyptrcea, Strombus, etc., the 

 tsenioglossate arrangement is found, represented by the for- 

 mula 2 or 3, 1, 1, 1, 2 or 3, the admedian teeth, however, being 

 very similar to the lateral. In other cases but a single median 

 tooth or the median with a single admedian on each side is 

 found, as in Fiosvs, Buccinum (the whelks), Nassa (Fig. 137, B), 

 Murex, Purpura, Oliva, Marginella, etc., forming the raclii- 

 glossate arrangement represented by the formulas — , 1, — , or 

 1, 1, 1. In Terebra, Conus, Pleurotoma (Fig. 137, C), and allied 

 genera the median tooth is absent, and the single admedian 



