56 THE DURATION OF LIFE. [I. 



and bivalves {Lamellibranchiatd). I could not incorporate them 

 in the text, for a number of necessary details as to the condi- 

 tions 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 destruction 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 communications, as a commencement to the life 

 statistics of the MoUusca. 



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

 after having produced the spawn from which the young de- 

 velope. These continue to grow until the following spring.' 



(2) ' The Succineae are mostly biennial ; Succinea putris prob- 

 ably triennial. Fertilization takes place from June till the 

 beginning of August, and the young develope until the autumn. 

 Succinea Pfeifferi and S. elegans live through the winter, and 

 the fact is proved by very distinct annual markings. Repro- 

 duction 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 

 Bulimus (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 propor- 

 tion 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. hispida, two to three years ; H. hortensis, H. nemoralis, H. 

 arbustorum, as a rule three years ; H.pomatia four years. Fer- 

 tilization is not in these species strictly confined to any one 

 time of year, but in the case of old animals takes place in the 

 spring, as soon as the winter sleep is over ; while in the two- 

 year-old animals it also happens later in the summer.' 



