GENERAL OBSERVATIONS. 885 



erous ; Cystideans, which culminated also in the Lower Silurian, and 

 had their last species in the early Devonian, though not their last spe- 

 cies in fact, since the depths of the Atlantic Ocean still contain Cys- 

 tids; Goniatites, which began in the Hamilton period of the Devonian, 

 and are unknown after the Carboniferous age. Many other instances 

 are given in the table beyond. The causes of such extinctions were 

 connected with a higher principle than that of mere physical catas- 

 trophe. 



The following table presents to the eye the history of many of the 

 genera, families, and tribes of Paleozoic species, showing, by means of 

 the narrow dark areas, the time of their commencement; the time of 

 their culmination (by the greatest breadth of the area) ; and the time 

 of their extinction in the course of the Paleozoic ages, or the fact of their 

 continuing to survive in after-time. Thus, opposite the word Polyps, 

 the area commences near the beginning of the Silurian, and increases 

 through the Paleozoic, but does not terminate there, since they exist 

 afterward ; the Cyathophylloid Corals begin with the Lower Silurian, 

 have their maximum in the Devonian, and only a few are known after 

 the Carboniferous. At the top of the columns, P. Pd. stands for Pri- 

 mordial Period ; and S., C, P., for Subcarboniferous, Carboniferous, 

 and Permian. 



9. Genera of the present time dating from the Paleozoic era. — The 

 number of lines connecting the past with the present is considerably 

 increased in the Carboniferous age. These lines are, however, only 

 long-lived genera, not species. The following are those which appear 

 to be determined with a good degree of certainty : — 



Lingula (?), Discina, Crania, Nautilus, Pleurotomaria, Rhyncho- 

 nella, Terebratida, Ostrea, Avicula, Pinna, Lima, Solemya, Leda, Nu- 

 cula, Dentalium, Chiton. They are all Molluscan. The first five 

 commenced in the Lower Silurian. It is to be acknowledged that 

 there may have been greater differences between the existing and 

 modern species of these genera than the shells have given reason to 

 suspect. In view of this, the older Lingulce have been of late called 

 LinguleUce, sufficiently great differences existing to excite the belief 

 that the animals were generically different. It is a remarkable fact 

 that there are no Radiate genera in this list. 



Besides the above, the genera Area and Astarte have been referred 

 to the Paleozoic ; but the species probably belong to other genera. 

 There are no genera of Articulates, unless it be the genus Spirorbis, 

 about which there is reason for much doubt. 



There are modern genera of Protozoans in the Paleozoic, and prob- 

 ably also of Diatoms ; and the number of such genera among these 

 protozoan and protophyte forms will probably be greatly increased, 

 when the species are further investigated. 

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