454 GEOLOGY. 



are replaced by intermediate forms. From this intermediate or transi- 

 tional assemblage the Helderberg fauna seems to have taken its origin 

 in part at least, and to have emigrated westward as the re-advance 

 of the sea permitted. 



The marked development of the mollusks. — As the physical con- 

 ditions in the originating tract gave a muddy, partially calcareous 

 bottom, the fauna developed in lines suited to such a habitat and, 

 as like conditions prevailed in the interior to which it emigrated, it 

 retained its original character as a silt-conditioned fauna. As a con- 

 sequence, mollusks and molluscoids greatly preponderated, and trilo- 

 bites were fairly developed, while corals and crinoids, lovers of clear 

 water, were relatively few and unimportant. The gastropods came 

 into unusual and quite peculiar prominence. The capulids, a rather 

 inferior or degenerate type, seem to have found singularly congenial 

 conditions, for they appeared in great variety of form. Two character- 

 istic forms of this group are illustrated in Fig. 206, a and b. The 

 group continued to play an important part in the faunas that occu- 

 pied the interior in succession during the following stages of the Devo- 

 nian period, whether because, having come in with the Helderberg 

 immigrants, they held their own notwithstanding the subsequent 

 invasions by other faunas, or because new forms came in with each 

 of these invading faunas, does not now clearly appear, though the 

 former seems the more probable. Especial interest attaches to this 

 group because it constitutes one of the characteristic features of the 

 Hercynian fauna in Europe, a fauna which has been the subject of 

 much difference of opinion but which is, with little doubt, the essential 

 equivalent of the American Helderberg. A number of gastropods 

 other than the capulids were present. 



The bivalves also blossomed out notably in large-winged forms, 

 the aviculids (Fig. 206, c). This was in sharp contrast to the Niagara 

 fauna, which showed but few of these forms. Like the capulids, thej^ 

 continued to be numerous through the succeeding Devonian faunas. 



Quite in contrast with the foregoing, the cephalopods had a very 

 meager development. From being one of the dominant types in the 

 Silurian, they fell to an insignificant rank. They were to rise again, 

 however, in a new form. 



The superior numbers of the molluscoids. — The inevitable brachio- 

 pods far surpassed all other classes in numbers. They were even more 



