THE TRIASSIC PERIOD 57 



less real. The number of pelecypods was relatively large, and the 

 majority of the genera were of the modern type, some being even 

 identical with living genera, but with these were mingled about half 

 as many that still bore a Paleozoic aspect. 



The change in the type of brachiopods. — The dominant brachio- 

 pod types of the late Paleozoic were outwardly distinguished by broad 

 forms and extended hinge lines, as the spirifers and orthids; the 

 narrower beaked or rostrate forms represented by the rhynchonellas, 

 formed a very respectable minority. In the Triassic period the ros- 

 trate forms Rhynchonella, Terebratula, and allied genera became the 

 predominant class, and have remained so ever since. The spire-bear- 

 ing forms (Spirijerina, etc.) were still present, though rare, and the 

 loop-bearing terebratuloids became much more conspicuous in the 

 Mesozoic faunas. 



The echinoids become the leading echinoderms. — Although the 

 echinoderms were not at all strongly represented in the Triassic fauna, 

 the period marks the transfer of echinoderm leadership from the crin- 

 oids to the sea-urchins. It also marks a structural change in these. 

 Beginning with the Triassic, the echinoids had twenty rows of plates 

 in belts of two rows each, as the invariable rule, whereas the Paleozoic 

 forms had more. At first they retained the previous regular pen- 

 tamerous symmetry, but later this gradually gave place to a bilateral 

 symmetry. Many of the Triassic forms were armed with club-shaped 

 spines. The crinoids were generally few, though sometimes locally 

 abundant. Starfishes and brittle-stars were present, but not abundant. 



The corals. — While corals were generally rare, in certain favored 

 localities, as at St. Cassian, they were rather prolific. While some 

 of them resembled the Paleozoic forms in being simple and cup-shaped, 

 the compound species took on the modern (hexacoralla) form, and 

 the compound Paleozoic (tetracoralla) type disappeared. These later 

 compound corals do not seem to have been derived from the Paleozoic 

 compound forms, but from some simple type. 



Other forms. — The marine arthropods seem to have been unim- 

 portant. Sponges were present in Europe, but have not been found 

 in America ; bryozoans were very few ; and f oraminif era were abundant 

 in favorable situations in Europe. All of these groups presented more 

 or less transitional or modern phases. 



While the general aspect of the Triassic marine faunas was emphati- 



