452 THE TRIASSIC PERIOD 



dita, may be selected as a few examples of the commoner genera. 

 The higher forms of the class, the Sinupalliata, are, however, still 

 rare. The Gastropoda are yet in a transition stage. Several genera, 

 such as Murchisonia, Loxonema, etc., here make their last appear- 

 ance, and mingled with them are the forerunners and earliest repre- 

 sentatives of modern types, such as Cerithium, Emargimila, etc. 



The Cephalopoda, and more particularly the Ammonoids, have 

 already acquired a wonderful degree of abundance and variety, 

 more than iooo species of the latter group having already been 

 described from the Trias. The ancient Nautiloid genus Orthoceras, 

 which ranges from the uppermost Cambrian through the whole 

 Palaeozoic group, persists into the Triassic system, but not later, 

 and numerous coiled Nautiloids with angulated and ornamented 

 shells- recall those of the Carboniferous. Of the Ammonoids some 

 still have the comparatively simple sutures of Goniatites, others, 

 like Ceratites, have slightly serrated sutures, while in the upper 

 Triassic occur some shells in which the complexity of the sutures 

 is carried farther than in any other known members of the group. 

 Only a few of this great assemblage of genera can be mentioned ; 

 especially characteristic of the Trias are : Tierolites, Trachyceras 

 (VIII, 3), Meekoeeras, Arcestes (VIII, 4), Ceratites, and Pinaco- 

 ceras. It is very interesting to observe that in the Trias occur, 

 though but rarely, certain unusual forms of Ammonoid shells, 

 which do not become important until the long subsequent time of 

 the Cretaceous period. Rhabdocei-as has a straight shell, Cochlo- 

 ceras one that is coiled in a high spiral like a gastropod, and in 

 Choristoceras the coils are open. The Cretaceous genera were 

 not derived from these Triassic anticipations, but are degenerate 

 members of many Ammonoid families. The Dibranchiate Cepha- 

 lopods, and especially the characteristic Mesozoic group of the 

 Belemnites, have their earliest and most primitive representatives 

 in the Triassic genera Atractites and Aulacoceras. 



The Vertebrata are of extraordinary interest, and if the Trias 

 had yielded only vertebrate fossils, it would still be apparent that 

 great progress had been made since the time of the latest known 

 Palaeozoic beds. The Fishes display this progress least of all the 



