INTKOUUCTIOX. Xxix 



a development of two whorls or so, the subsequent shell-growth 

 after birth is well marked by its different texture ; the first whorls 

 are generally smoother and are distinguished as the protoconch. 



It is seen that the organs of generation in these creatures are 

 very complicated and present great variation, not only shown in 

 different genera, but even in detail in different species of the 

 same genus. It is only to be expected that the more complicated 

 an organ becomes, and the greater the number of its component 

 parts, the play and possibility of variation in those parts increases 

 in proportion. This diversity of structure is rendered still more 

 difficult to observe, owing to the great changes that take place in 

 the reproductive organs as the pairing-season approaches, also 

 artificially by the means adopted to preserve the animal. If, for 

 instance, it is put into too strong spirit, gi-eat shrinking and 

 hardening ensues, and the parts assume a shape and size 

 very unlike their appearance in life. All this must be allowed 

 for in descriptions. Perhaps too much has already been written 

 on the form of such an organ as the spermatheca or sperm-sac, 

 sometimes empty, at other times distended with one or more 

 spermatophores. 



The Mollusca form a very distinct and highly organized division 

 of the Invertebrates. They were very early inhabitants of the 

 globe; their remains are found in the oldest stratified rocks, their 

 shells often most beautifully preserved. It is the knowledge of 

 the animals of their living representatives which gives us an insight 

 into the conditions under which the difterent formations were 

 deposited. It is remarkable to note ho\^' highly organized they 

 were so far back in time and how little has been the change since 

 then, scarcely more than generic. 



Marine forms are more abundantly represented, and naturallv 

 so, than the laud and freshwater. Probably the earliest fresh- 

 water form is the ArcJianodon, which closely resembles the modern 

 Anodonta (Swan Mussel) and occurs as far back as the Devonian. 

 At the close of the same epoch the first land-snails, Strophites and 

 Dendropvpa, allied to the Chrysalis Shells (Pupidse), have been 

 found in the Plant-beds at St. John, Kew Brunswick. The Coal- 

 Measures of the succeeding Carboniferous period have yielded 

 further species of Doidro^mpa, a small land-shell (formerly referred 

 to Zomtes) closely allied to the liviug Fi/ramidida if not* identical 

 with that genus. Also from the same beds come the oldest brackish- 

 water snail and freshwater snail belonging to the existing genera 

 AmpidJaria and Fhysa respectively, as well as the first repre- 

 sentative (Zcqitticlms) of the Auriculid* (the most primitive of the 

 Pulmonates), and DaivsoneUa the earliest example of the terrestrial 

 Hehcindai. Other freshwater mussels (Unio) appeared in the 

 Trias, but the greater number of freshwater snails are first known 

 from the Jurassic : PJanorhis, Valvata, Melania (doubtfully frou) 



