XIV CONTENTS. 



PAGE 



of the bivalves, 349, 350 : fry of fresh water mussels, 350 : dioecious 

 Mollusea, 350 362 : the cryptogamous kinds, 351 : the phsenogamous 

 kinds, 351 : the Phytophaga, their ova, 351 : viviparous Phytophaga, 352 : 

 Zoophaga, their ova, 353 ; their egg-capsules, 354 361 : sexes of the 

 Cephalopods,361 ; their ova, 362 : hermaphroditical Mollusea, 363369 : 

 the love-darts of the Helix, 364, 365 : the spawn of the aquatic species, 

 365 367 : the ova of the terrestrial kinds, 367 369 : viviparous kinds, 

 370 : affection of the Mollusea to their young, 370, 371 : their meta- 

 morphosis, 371374. . . . . . . . 339 



LETTER XXI. 



Age of the Mollusea, 375, 376 : their tolerance of wounds, 376 : their 

 reproduction of amputated organs, 377 379 : their diseases, 379 

 384 : the mite of the snails, 379381 : the gordius of the fresh water 

 snails, 381 : the leech of the bivalves, 381 : and their parasites, 382 : 

 the parasite of the Octopus, 383384. . . . .375 



LETTER XXII. 



The structure and formation of shell : analogous to the skin, 385 : 

 not peculiar to the Mollusea, 386 : composition of shell, 387 : Reaumur's 

 theory of its formation, 387 : opposed by Herissant, 387, 388 : re- 

 searches of Mr. Bowerbank, 389 393 : researches of Dr. Carpenter, 

 393399 : cellular structure, 394 : membranous structure, 394 : 

 nacreous structure, 395 : tubular structure, 395, 396 : cancellated struc- 

 ture, 396 ; formation of shell, 397399 : growth of shells, 400, 401 : 

 correspondency between the shell and its animal, 401, 402. . . 385 



APPENDIX. 



Of shells, apparently similar but belonging to different genera, by 

 J. E. Gray, 403-407. . . . ..403 



LETTER XXIII. 



The formation and structure of shells : first formation of shells, 408 

 411 : the nucleus, 409, 410 : the periostracum, 410, 411 : the exter- 

 nal form of shells and their variations, 411425 : position of the shell 



