79 



less mussels (Modiomorpha), which at once began to settle 

 down and appropriate the ground for themselves. These 

 new-comers may have appeared from without, or they may 

 indicate the sudden increase in numbers of the few forms 

 which existed during the preceding ages. In any case, their 

 development was rapid and complete, and so was their 

 extermination. Thus was formed the mussel-bed which now 

 appears in the sections twenty-five feet below the Encrinal 

 limestone. After the mussels had virtually disappeared, 

 except for a few stragglers, which remained on the scene of 

 their former occupancy, brachiopods once more appeared in 

 numbers, and continued thus for a very long period of time. 

 At one time numberless individuals of Athyris spiriferoides 

 appeared on the scene, and their appearance seems to have 

 driven out most of the other forms of life, as recorded in the 

 Athyris spiriferoides bed. Later, however, these returned 

 again, but before they finally re-established themselves, some 

 unexplained changes took place, which temporarily estab- 

 lished conditions similar to those which prevailed during the 

 later Marcellus epoch. The two pteropods, Stvliolina 

 fissurella and Tentaculites gracilistriatus, became the sole 

 occupants of the water, and as they probably were pelagic 

 animals, their occurrence in these shales may be explained by 

 the assumption of the existence of currents, which carried 

 them in from the open sea. 



At last, when the Hamilton age was near its end, the rich 

 fauna of the Demissa bed appeared. This sudden develop- 

 ment of forms, many of them appearing for the first time, 

 can only be explained by supposing immigration to have 

 taken place. An interesting feature of this bed is the 

 occurrence of two brachiopods, Strophe odonta plicata and 

 Spirifer asper, both of which belong normally to the Hamil- 

 ton fauna of Iowa. This fact would indicate, that at some 

 time during the later Hamilton age, fairly uniform conditions 

 extended westward from this region to Iowa, and perhaps 

 bevond, 



