518 SLUDIES HOR SLODENTS 
their larval and adolescent history, the protoconch and early 
chambers being enveloped and protected by later stages of the 
shell. And by breaking off the outer chambers the naturalist 
can in effect cause the shell to repeat its life history in inverse 
order, for each stage of growth represents some extinct ances- 
tral genus. These genera appeared in the exact order of their 
minute imitations in the larval history of their descendants, and 
by a comparative study of larval stages with adult forms the 
naturalist finds the key to relationships, and is enabled to 
arrange genera in genetic series. They were all marine, never 
parasitic, and so with them there is no obscuring of the record; 
also in the Mollusca generic and specific characters show in the 
shell better than in the soft parts; so the classification of fossil 
ammonites is just as good as that of living shellfish. 
Although genera appeared in the order of corresponding 
larval stages, they did not disappear in the same order; and so 
their survival under favorable conditions is liable to make con- 
fusion in the record, if one depends wholly on the study of 
series of adults. Such forms, for instance, as Styrites, Tropicel- 
tates, Miltites and others that are now known only in the Karnic 
zone of the Upper Trias are undoubtedly such survivals, for they 
still have simple goniatitic sutures, very little ornamentation, 
and in general are more like Lower Triassic ammonites than 
members of the Zvopites subbullatus fauna. The stray Tzrolites 
foliaceus, which appears in the Alps and in California in this 
same fauna, is another survival of a Lower Triassic type, but for- 
tunately we do know 7Z7rolites in the horizon where it belongs. 
If this were not the case the naturalist would be very much puz- 
zled at finding 7rachyceras of the Karnic horizon going through 
a Lirolites stage in its early youth. 
One great drawback to this work is that the ammonite faunas 
of the various ages have been classified by different specialists 
and on different principles, but all artificial. Thus the Triassic 
ammonites are divided into Leiostraca (smooth shelled), and 
Trachyostraca (rough shelled), a classification that cannot be 
extended even to Jurassic groups. The Trachyostraca are fur- 
