400 GEOLOGY. 



some had fallen, while all had undergone a change. The more conspicu- 

 ous features of these changes claim attention. 



Echinoderms : the Mastoids. — A distinguishing feature of the Silurian 

 fauna was the rich and varied development of the echinoderms, involv- 

 ing at once the rise or the decline of old forms and the introduction 

 of new ones. To the five classes present in the Ordovician, there was 

 now added the blastoids, and the full list of classes adapted to fossili- 

 zation was completed. The new class consisted of symmetrical, bud- 

 like forms, with a calyx of a definite number of plates, very regularly 

 and beautifully arranged in pentamerous order, but without true arms, 

 as shown in Fig. 187, i. Only two genera of blastoids, so far as known, 

 were as yet present, but in the two succeeding periods a rapid develop- 

 ment was achieved, followed by a swift decline. 



The rise of the crinoids. — The appearance of the blastoids was 

 a mark of advance ; but the dominant feature was the rise of the crin- 

 oids. They attained such abundance in certain congenial localities 

 that their fragments formed the main substance of the limestone, 

 These spots became veritable " flower-beds " of " stone lilies/ ' and 

 certain localities, as Lockport, N. Y., Waldron and St. Paul, Ind., Ra- 

 cine, Wis., Chicago, 111., Gotland, Sweden, and Dudley, England, have 

 become noted as peculiarly rich crinoidal fields, where beautiful and 

 varied forms grew in groves, as it were. The assemblage of species 

 at each of these noted localities had its own peculiarities, but the genera 

 were usually much the same. A few Silurian crinoids are shown in 

 Fig. 187, a, b, c, d, e, but no such limited selection of figures can do 

 justice to the variety and beauty of form they displayed. 1 The wide 

 differences of those illustrated, however, gives some hint of the develop- 

 ment of the class. Among the notable features was a large projecting 

 tube whose apparent function was to carry the used waters of the 

 body cavity away for discharge at a distance from the food grooves, 

 with an apparent hygienic purpose. In certain cases, this tube was 

 curiously expanded above, the plates spreading into a broad umbrella- 

 like canopy with the apparent purpose of still more effectively pre- 

 venting the mingling of the used and unused water. A new feature 

 was the consolidation of the plates at the base of the calyx, imparting 



1 For a fine series of illustrations see the classic work of Wachsmuth & Springer, 

 The North American Crinoidea Camerata, Mus. Comp. Zool., Cambridge, Mass., Vol 

 XXI, 1897. 



