56 GEOLOGY. 



date themselves to the scant opportunities offered. This is shown best 

 in the development of the land and fresh-water life, but it is also ex- 

 pressed in the marine life. 



The cephalopods again in leadership. — The mosi conspicuous fea- 

 ture was the re-ascendancy of the cephalopods in the form of the 

 ammonites, which had a marvellous development during the period, 

 reaching a thousand species. Their evolution was made the more 

 notable because their structural changes were conspicuous and showed 

 declaredly the advance' of each stage upon the preceding. While the 

 straight orthoceratites, the simplest type of the cephalopods, still 

 persisted with notable tenacity, and the simplest coiled nautiloid 

 forms with plane septa also persisted, the closely coiled, intricately 

 sutured forms overwhelmingly predominated. There also appeared 

 at this time the first of the known cephalopods of the cuttlefish type 

 (Dibranchiata) . The deployment of the cephalopods was therefore 

 varied and comprehensive to a degree never reached before, and per- 

 haps not much surpassed afterward, although the culmination of this 

 evolution took place in the succeeding period. The old forms, how- 

 ever, the orthoceratites and even the goniatites, make their last appear- 

 ance in this age, and were not participants in the culminating fauna 

 of the Jurassic. The remarkable commingling of old and new forms, 

 orthoceratites, nautiloids, goniatites, ceratites, and ammonites, with 

 its suggestiveness relative to derivations and transitions, makes this 

 one of the most instructive assemblages in the history of the cephalo- 

 pods. 



Old and new gastropod types. — A similar commingling and tran- 

 sitional aspect was presented by the gastropods. The Paleozoic gas- 

 tropods possessed apertures which were " entire "; that is, nowhere 

 drawn out into a tube for the reception of the siphon. Sometimes 

 there was a recess or slit in the aperture, but no tube or canal. The 

 progressive branch of the Triassic gastropods, however, developed 

 such tubes and originated the canaliculate class. By means of the 

 canaliculate shell, the used waters from the body chamber were carried 

 a longer distance from the orifice by which fresh waters entered the 

 chamber, and thus served a hygienic function. 



The transition and rise of the pelecypods.— The Triassic bivalves 

 do not show the transition from the old to the more recent by as con- 

 spicuous features as the cephalopods and gastropods, but it was scarcely 



