No. I.] LARVAL STAGES OF SCHLOENBACHIA. 239 



of a number of fossil species, and have compared the growth 

 stages of these with the history of the group. Here, again, 

 comes in the difficulty that ontogenic series can be obtained 

 only by putting together in a row a number of distinct individ- 

 uals of various sizes, with the chance of making mistakes in 

 identification increasing as the specimens grow smaller and 

 have fewer characteristic marks. Very naturally, closely re- 

 lated species become more alike as we go down to the younger 

 stages, until it is impossible to tell which species the very 

 young larvae belong to. Of course some species have specific 

 characters thrown back by acceleration until even the earliest 

 larval stages are recognizable, but usually specific characters do 

 not appear until the adolescent period is well advanced. This is 

 true of all marine invertebrates that go through a larval period. 



Of living molluscs the gastropods and the pelecypods offer 

 the same difficulties as the brachiopods, and have been much 

 less studied ; even of the common oyster not all the larval 

 stages are known, and no other mollusc has been so closely 

 studied as that has. 



Life history of cephalopods. — The chambered cephalopods 

 offer the best means of comparing ontogeny with phylogeny, 

 although the one available living form. Nautilus, belongs to the 

 old unspecialized group of nautiloids that has changed little 

 since its origin. Here, again, we are thrown back on paleon- 

 tology, but this time the difficulties are not so great, for there 

 is a great group of cephalopods, the ammonoids, that has left 

 in the stratified rocks abundant materials for study. 



The ammonoids branched off from the nautiloids in the 

 Upper Silurian or the Lower Devonian, at first small, simple, 

 and rare, but they developed rapidly, until by the end of the 

 Devonian all the groups of goniatites were already present. 

 These increased steadily in numbers, size, and complexity, and 

 during the Carboniferous gave rise to the first simple ammo- 

 nites ; these latter are a distinctly, although not exclusively, 

 Mesozoic race, which developed with wonderful rapidity from 

 the first rare members into numerous families, hundreds of 

 genera, and thousands of species, reaching their acme in the 

 Jurassic. In the Cretaceous they gradually declined, dropping 



