On the Form, Growth, and Construction of Shells. 19 



hatched, which, forms the apex or the umbo of the adult shell. 

 But the shell, in its subsequent growth, frequently differs 

 entirely from the embryo, both in form and colour. The shell 

 of the infant Cymba olla is large and very irregular ; in 

 Magilus, and in Patella, it is spiral, but afterwards becomes 

 tubular in the former, and tent-shaped in the latter. In the 

 Nudibranchiata the shell of the embryonal mollusk is shed at 

 an early age, and never replaced. Many of the minute shells 

 found, as well fossil as recent, are probably only the .fry of 

 larger species, and require great caution in their determination. 



The size of the adult shell is often characteristic of the 

 species, but this is by no means uniform. The author has 

 frequently seen specimens of Cyprcea turdus, equally adult, 

 measuring three-quarters, to one and a half inches, but the 

 dwarf varieties are more common than the giants. 



Law of Alternation and Periodicity. — In summer and winter 

 land- snails cease to grow. The snails of the first year, hatched 

 in the spring, usually attain half their growth in the autumn of 

 the same year, and their maturity in the following spring. 

 There is always a stronger line of growth or conspicuous mark 

 on banded and garden snails, and in some a rib inside 

 strengthens the rim of the half-grown shell. 



In the writer's cabinet are two examples of Helix aspersa, 

 the common garden snail, which are grown together. They 

 had passed the winter months hybernating in the same retreat, 

 the one being probably nine months old, the other fifteen 

 months. The younger snail died, having, no doubt, been killed 

 by the frost ; the survivor crept forth in the spring, bearing 

 his deceased but irremovable companion, firmly cemented upon 

 his back. As the living snail continued to grow, he came in 

 due course around his own axis to the spot where the dead 

 snail still remained fixed, and being unable to disconnect it, he 

 formed his new shell over his attached companion, and thus in 

 death they remain united. 



Sea-snails certainly take many seasons to attain maturity, 

 even supposing them to grow twice a year. Dredging is 

 usually only carried on in the spring and summer months ; yet 

 a large proportion of the mollusca taken are immature. 

 PJulima grows a whorl at a time, then thickens its lip and 

 rests ; ultimately a straight line is found down one side of the 

 shell, caused by the coincidence of these "rests." 1 ' In Banella 

 the line of " rests" is also coincident ; but as it only grows 

 half a turn between each, there are two rows down the spire. 

 In Triton (the shell usually represented being blown as a horn 

 by sea-deities attendant upon Neptune and Amphitrite) the 

 "periodic mouths" form alternating nodes up the spire to the 

 slender apex. The Mimcidce are extremely varied in form by 



