1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 813 



genus Helix, it is now regarded as a member of the Helicinidee, 

 but differs from the latter in possessing a large basal columellar 

 callosity covering the umbilical region. The operculum is 

 apparently unknown. The affinities of Dawsoniella must be 

 considered doubtful, but in any case its resemblance to the 

 Helicinidfe is to be i-egarded as due to convergence, and not to 

 natural affinity. In the first place, it is highly improbable that 

 if Helicinidse had existed from the Carboniferous onwards, no 

 trace of their remains should have been discovered in secondary 

 and earlier tertiary deposits. In the second place, the Helicinidse 

 ■are unquestionably derived from the Neritidfe, and, as we have 

 ■seen, the Neritidte were not established in Carboniferous times. 

 It is interesting, however, to note that the genus NaticojJsis 

 {Trachydomia) occurs in the same formation as Daivsoniella, and 

 it seems probable that we have here an interesting case of parallel 

 development. If Ncdicopsls, a marine form, was the forerunner 

 of the later Neritidfe, it would seem to have given rise in 

 Carboniferous times to the terrestrial Dawsoniella, just as the 

 mai-ine Neritidae have given rise in later time to the terrestrial 

 Helicinidse ; and the ancestry and conditions of life being similar, 

 the two terrestrial forms acquired such a similitude that their 

 ■shells have been classified together in the same family. 



Though the several genera of recent ISTeritacea have been 

 studied with minute care from a systematic point of view, we 

 have no very satisfying account of their habits. The genus 

 Nerita is confined to tropical or subtropical seas, and if we accept 

 for the moment the limitations of the genus as defined by con- 

 chologists, all the species are marine and are found for the most 

 part between tide-marks, clinging like limpets to the rocks. 

 Some of the more brightly coloured species live on coral-banks. 

 It has been remarked by several ti'avellers that they are capable 

 of enduring a considerable amount of exposure to the air. Thus 

 Qnoy and Gaimard (36) were surprised to see Keritce attached to 

 black rocks under the full glare of a tropical sun, without 

 apparent injury, and they observed that these animals always 

 retained a few drops of water in their shells which they ejected 

 when forcibly torn from their attachment. C. B. Adams observed 

 a West-Indian species living in crevices in the rocks between the 

 tide-marks at the height of three-quarter ebb-tide, and the young 

 forms were even higher up, attached to rocks and stones which 

 were only wetted by spray. Practically nothing is known of the 

 breeding-habits of Nerita, and in view of the complexity of 

 the accessory genital organs, especially in the female, observations 

 on this point are very much to be desired. 



As to the extent to which difierent species of Xerita are 

 tolerant of brackish or even of fi'esh water, very little information 

 of a satisfactory character is forthcoming. Many species are 

 recorded from bays at the mouths of rivers or from estuaries, 

 where there must be a considei'able admixtui'e of fresh and salt 

 Avater. Nerita lineata Chemnitz is i-ecorded as ascending the 



