'1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 847 



JV. jyUcata Linn. This species is widely distributed in the Indian 

 and Pacific Oceans. My specimens were obtained, through 

 the kindness of Mrs. G. B. Longstaff, from Ceylon. 

 iV. onelanotraga E. A. Smith. Specimens of this Australian 

 species were kindly collected for me by Mr. Geoffrey W. 

 Smith, of New College, Oxford. 



Paranerita. 

 -iV. variegata Lesson. From the East Indies and Polynesia. I 

 am indebted to Mr. E. A. Smith for specimens of this species, 

 which is synonymous with JV. gagates Mbrch. 



]^. gagates Lam. From Mauritius. 



iV. longispina Recluz. From Mauritius. My specimens of the 

 last-named two species formed part of the collections of 

 the Oxford Museum, and in the same collections I found a 

 number of fairly well-preserved spirit-specimens of an 

 unknown species from Fiji, wdiich I could not determine 

 because the animals had been extracted from their shells and 

 w^ere not accompanied by any note of identification. 



Neritina. 

 JSF.Jluviatilis Miiller is common in the Isis and Cherwell and in 

 the smaller streams near Oxford. 



In dealing with the general anatomy of these species I shall 

 ■chiefly occupy myself with a description of those features in 

 which they differ from one another, but it will also be necessary 

 to enter into some details about organs in which they resemble 

 • one another, bvit in i-espect of which there is disagreement among 

 previous authors. 



External Features. 



Septaria, as has been shown, has undergone so much reduction 

 of the visceral spire that it has acquired a secondary external 

 symmetry, but the three sections JVerita, Paranerita, and JS^ej^i- 

 tina retain to a much greater extent the primitive gastropod 

 asymmetry. It is obvious, however, that they are tending 

 towards a secondary external symmetry, the viscei'al spire being 

 relatively small and making scarcely more than half a tui'u. 

 The shell-muscle or columellar muscle is a striking feature in the 

 Neritidge. It is always paired and subsymmetrical. Both muscles 

 . ^re coarsely fasciculated ; that of the right side is somewhat the 

 stouter, that of the left side somewhat longer in an antero- 

 posterior direction. The two muscles are attached to the right 

 and left inner surfaces of the shell within the area, and their 

 ■impressions are not easy to see in those shells in which -the area is 

 strongly toothed or highly developed. The muscle-fibres of each 

 side converge downwards and inwards from their surfaces of 

 .attachment to the shell and pass into the powerful muscular 

 -mass forming what is really the anterior end of the opercular 



Proc. Zool. Soc— 1908, No. LIV. 54 



