534 PALEONTOLOGY OF ILLINOIS. 



FENESTELLA Lonsdale, 1839. 



(Murchison's Silurian System, p. 677.) 

 (For generic diagnosis see page 395.) 



Very much indeed might be written upon this remarkably 

 prolific group of Bryozoa, but, as space is limited, I must re- 

 frain. A few remarks however seem to be called for. 



In looking over the one hundred or more described species of 

 Fenestella,*, we see that the original infundibuliform zoarium, 

 which prevailed exclusively among the species of the Cincinnati 

 and Clinton groups, gradually gave way in the Niagara and 

 Lower Helderberg deposits until in. the Upper Helderberg and 

 Hamilton groups it is far less common than the flabellate zoar- 

 ium, while in the Lower Carboniferous groups and Coal Meas- 

 ures the original form of growth is almost unknown. The 

 number of the flabellate species is also so much greater that it 

 is evident that the genus had not attained its most typical de- 

 velopment before the beginning of the Upper Helderberg. The 

 earliest infundibular species are really the ancestors of the whole 

 family, and in the Niagara and Lower Helderberg groups sev- 

 eral species show departures from the original simple forms 

 along lines of variations which did not attain full development 

 until well into Devonian times. Thus in F. acmea Hall, F. 

 thyene Hall, and other species we see the progenitors of Semi- 

 coscinium. Unitrypa was developed from such forms as F. qua- 

 drula Hall, and F. precursor Hall, while two- and three-ranged 

 Polyporse began early. No intermediate forms are as yet 

 known showing the descent of Isotrypa, of which the Niagara 

 F.? amhigua Hall, is already a fully developed species. 



These facts are thrown out rather in the light of suggestions 

 to students who are desirous of working out the life history of 

 the family. My own investigations in this line have scarcely 

 begun, yet the results have already proved most gratifying and 

 sometimes surprising. 



* Many more have been described as Fenestella that belong to Polypora, Semico- 

 scinium, Unitrypa and other genera of the family. 



