408 GEOLOGY. 



The initial decline of the trilobites. — No new families of trilobites, 

 so far as known, were introduced during the period. Some new genera 

 were added and many new species, but these did not offset the dis- 

 appearance of old ones, and tlie class, though still important, may be 

 regarded as having already entered upon its decline, so far as preva- 

 lence was concerned. This was not true apparently in respect to the 

 rank of certain leading types, which continued to advance in organi- 

 zation. The highest Silurian forms were structurally equal, and per- 

 haps superior, to any that preceded. The causes which induced the 

 decline in numbers do not seem to have been incompatible with indi 

 vidual and generic progress. Singularly erratic developments of form 

 attended the decline of the trilobites, some features of which are shown 

 in Fig. 187, y, z, and it. Less erratic types of trilobites are shown in 

 Fig. 187, uu, vv. 



Other crustaceans. — In addition to the trilobites, ostracodes were 

 not uncommon, and their little bivalve shells are sometimes abundant 

 as fossils. The ceratiocarids were also present. In view of the remark- 

 able development of a distinctly crustacean fauna in the last stages 

 of the period, the scantiness of these crustaceans in the Mid-Silurian 

 epoch is notable. 



Other marine invertebrates. — There was a most prolific field of 

 sponges in western Tennessee, where the conditions were not only 

 congenial to their growth but favorable to their preservation. The 

 most abundant genus was Astylospongia (Fig. 187, k), a more or less 

 spherical form characterized by notably grooved surfaces. A saucer- 

 shaped form, Astrceospongia meniscus (Fig. 187, /), was also abundant. 



Mr. G. F. Harris has recently found that sponges form the nuclei 

 of chert nodules in the Chicago region. The genera Astylospongia, 

 Aulocopina, and Hindia have been identified. 



The peculiar Receptaculites family, whose zoological affinities were 

 long in doubt, was still present, but chiefly represented by Ischadites. 



The graptolites had lost much of the importance they possessed 

 in Ordovician times, and in the course of the period approached extinc- 

 tion. Sea worms are recognized through their jaws, tracks, and bur- 

 rows, and by the calcareous tubes which some of them secreted. 



The scant record of the vertebrates. — In the earlier and Mid-Sib 

 urian deposits very few relics of fishes and fish-like organisms have 

 been found, and these few are very imperfect. Near the close of the 



