I92I. 



N. Annandale & R. B. S. vSewell : Vivihara. 



245 





A. 



to one another and to the rows of chaetae, but forming onlj" very 

 tine ridges with minute irregular processes or serrations. These I 

 shall call the secondary 



PERIOSTRACAL RIDGES of 



the embryonic shell. 

 Finally, still finer oblique 

 longitudinal or vertical 

 lines can be detected un- 

 der a powerful lens, run- 

 ning across the spiral 

 lines in such a wa^- as 

 to form a delicate reticu- 

 lation with rhomboidal 

 meshes. That all this 

 ornamentation is mainly 

 periostracal can be proved 

 by dissolving the calcare- 

 ous matter of the shell 

 with weak acid. The 

 lines and chaetae remain 

 intact. 



In the other species of 

 Viviparidae examined the 

 periostracal ornamenta- 

 tion is essentially the 

 same, but in several, the 

 test-scul)iture being more 

 highly developed, the 

 chaetae are given greater 

 prominence. I will dis- 

 cuss this point later. In 



the embryonic shell of V . diisimius (fig. 10), two of the second- 

 ary periostracal ridges bear minute chaetae considerably finer 

 and shorter than those of the three primary rows but essentially 

 similar in structure. These secondary rows are situated between 

 the first and second primary rows and above the latter. 



In some species the periostracal ornamentation of the embryo- 

 nic shell becomes obsolescent at an early age, but in all I have 

 examined the peripheral row of chaetae is continued, at any rate in 

 some individuals, on to the bo^\'-whorl of the full-grown shell, 

 and the apparent disappearance of the chaetae of the other rows is 

 more apparent than real. These structures are of extreme fragi- 

 lity and in a comparatively heavy organism such as an adult 

 Vivipara are liable to be rubbed off at a touch. In a nearly full- 

 grown V . bengalensis, in which the shell is receiving its final addi- 

 tion, I have found that the three rows of chaetae are still produced, 

 but disappear almost as soon as they are formed. In V . dissimilis 

 traces of the embryonic periostracal sculpture are more persistent 

 and the basis of the five rows of chaetae can frequently be detected 

 in the form of fine punctures. Even in the adult of L. Iccythis the 



J. 10. — Kmbryonic shell of 

 mill's (Muller). 



Oblique view of the whole shell at birth. 



showing lines of chaetae (magnified). 

 Pari of the surface of the body-whorl of 



the same shell more highl)' magnified. 



1 = uppermost primary row of chaetae ; 



2 = middle primary row of chaetae ; 



3 = peripheral row of chaetae ; 



i' 2' = secondary rows of chaetae. 



