APPENDIX. 55 



i 



NOTE 5. THE DURATION OF LIFE IN INDIGENOUS TERRESTRIAL 

 AND FRESH-WATER MOLLUSCA. 



I am indebted to Herr Clessin the celebrated student of our 

 mollusca for some valuable notes upon our indigenous snails and 

 bivalves (Lamellibranchiata). I could not incorporate them in the 

 text, for a number of necessary details as to the conditions of life 

 are at present entirely unknown, or are at least only known in a 

 very fragmentary manner. No statistics as to the amount of de- 

 struction suffered by the young are available, and even the number 

 of eggs produced annually is only known for a few species. I 

 nevertheless include Herr Clessin's very interesting communica- 

 tions, as a commencement to the life statistics of the Mollusca. 



(i) ' Vitrinae are annual ; the old animals die in the spring, after 

 having produced the spawn from which the young develope. These 

 continue to grow until the following spring.' 



(z) ' The Succineae are mostly biennial ; Succinea putris probably 

 triennial. Fertilization takes place from June till the beginning of 

 August, and the young develope until the autumn. Succinea Pfeif- 

 feri and S. elegans live through the winter, and the fact is proved 

 by very distinct annual markings. Reproduction takes place in 

 July and August of the following year, and they die in the autumn. 

 They continue to grow until their death.' 



(3) ' The shells of our native species of Pupa, Clausilia, and Buli- 

 mus (with the exception of Bulimus detritus) show but faint annual 

 markings. They can hardly require more than two years for their 

 complete development. The great number of living individuals 

 with full-sized shells belonging to these genera, as compared with 

 the number which possess smaller shells, makes it probable that 

 these animals live in the mature condition longer than our other 

 Helicidae. I have always found full-sized shells present in at 

 least two-thirds of the individuals of these genera characterized by 

 much-coiled shells a proportion which I have never seen among 

 our larger Helicidae. Nevertheless direct observations as to the 

 length of life in the mature condition are still wanting.' 



(4) ' The Helicidae live from two to four years ; Helix sericea, H. 

 kispida, two to three years ; H. kortensis, H. nemoralis, H. arbustomm, 

 as a rule three years ; H. pomatia four years. Fertilization is not 

 in these species strictly confined to any one time of year, but in the 



