g6 SMITHSONIAN INSTITUTION 



ture, being that of a strong anal side and solid walls, is specialized in the direc- 

 tion of the Camerata, and the family ran its course from opulence to poverty 

 and ultimate extinction very much as did the Camerata. The Taxocrinidae, 

 on the other hand, represent the true Flexibilia type, comparable to stages in 

 the ontogeny of the living crinoids, and its progress was one of steady increase 

 to a maximum in the Mississippian, or Mountain limestone, period of the 

 Lower Carboniferous, followed by extinction at the close of that age. Both 

 the Sagenocrinidae and Ichthyocrinidae maintained a fairly even course, ex- 

 cept for the entire absence of the former in the Devonian; this fact empha- 

 sizes the imperfection of the geological record in reference to this group, for 

 the succession from the Silurian to the Carboniferous forms in this family is 

 so evident that it is' impossible to imagine an actual break; and it is to be hoped 

 that some new Devonian locality will yet furnish the missing connection. The 

 rapid rise and great expansion of the Flexibilia in the Silurian accord with the 

 development among the other crinoids, as well as in many other forms of life 

 which flourished abundantly in an age characterized by conditions favorable 

 to most forms of shallow water life. Taken as a whole, however, the acme of 

 the Paleozoic crinoids was attained in the Lower Carboniferous. 



The American species of the Flexibilia exceed in number those from 

 Europe by a proportion of 60 per cent against 40; the disparity is not so great 

 as in the Camerata or in the Inadunata. The maximum of European species 

 was reached in the Silurian, which has produced 42 out of the total of 71 ; less 

 than a third of that number are from the Carboniferous. In America the 

 Silurian species number 29, and the Carboniferous 57. 



In the Silurian Flexibilia there is unquestionably a great similarity be- 

 tween the American and European faunas, which has been emphasized by the 

 discoveries of recent years in this country. Pycnosaccus, Anisocrinus, Clid- 

 ochirus, Sagenocrinus, Protaxocrinus and Gnorimocrinus, all formerly sup- 

 posed to be strictly- British or Swedish genera, now prove to be represented by 

 good species in the American rocks ; and of the three species of my new genus 

 Hormocrinus, one comes from each of these countries. In the highly special- 

 ized and hitherto little known Pycnosaccus, of which I now have abundant new 

 material, some of the specimens from Tennessee can scarcely be distinguished 

 from those of Dudley and Gotland. Three rare and specialized European 

 genera of the other orders have also been found in the American Silurian, 

 among them the very remarkable Crotalocrinus ; while the almost equally re- 

 markable American form, Pctalocrinus, has been discovered in Sweden. Of 

 the 17 Silurian genera of the Flexibilia, all but one are represented in the 

 European rocks, while 7 are unrepresented in America, most of them being 

 extremely rare or local in their occurrence; thus 9 of the genera are common 

 to both areas. Some of these facts (not all being before known), together with 



