456 GEOLOGY. 



numerous in genera and species than in the Silurian period, but did 

 not develop such tangible peculiarities as the gastropods and pelecy- 

 pods. Illustrations of leading forms will be found in Fig. 206, d to q. 

 There were many members of the Orthis family (Fig. 206, /, k, and I) 

 which gave a somewhat Silurian facies to the fauna, but a study of 

 the details of these forms shows the initiation of the Devonian type 

 of development. The members of the Strophomena family (Fig. 206, e, /) 

 were also very abundant and much diversified, a foreshadowing of 

 their much greater development in the succeeding Onondaga and 

 Hamilton epochs. The spirifers (Fig. 206, d) had come to be a striking 

 feature of the assemblage. They had made a very notable advance 

 upon their relative place in the Silurian fauna. Several of the Helder- 

 berg spirifers were clearly successors of Silurian species and represent 

 consecutive evolution, but a larger number were not so much lingering 

 representatives of known earlier forms as initial forms of new types 

 which were to be more fully developed in the succeeding Devonian 

 faunas. The loop-bearing brachiopods appear at this time in the 

 genus Rensselceria (Fig. 206, h) which reached its maximum develop- 

 ment in the next epoch. 



The bryozoans were fairly developed and foreshadowed coming 

 types, rather than represented past types (Fig. 206, r). 



The aberrant tendency of the trilobites. — The trilobites dis- 

 tinguished the epoch by their notable tendency to sport, a tendency 

 which reached its climax later. They took on strange and distinctive 

 forms, one of which is illustrated in Fig. 206, v. The peculiar shapes 

 of the head parts and their remarkable ornamentation seem inexpli- 

 cable unless they had an esthetic function, which would presuppose 

 a high mental development. But, whether explicable or not, the 

 development of the ornamented and erratic forms represents a stage 

 in the evolution of the class quite worthy of note, not only as character- 

 istic of the Devonian evolution, but as a precursor of the approaching 

 disappearance of the whole group. It will be seen later that as the 

 trilobites neared the end of their career their forms became aberrant. 



The scantiness of the crinoids and corals. — The crinoids had fallen 

 from their former high estate in the Silurian to an insignificant place, 

 not fatally and finally, but temporarily, on account, we may sup- 

 pose, of the rather turbid waters which the shales and shaly constitu- 

 ents of the limestones indicate. The cystids (Fig. 206, t) suffered less 



