REPORT OF THE STATE PALEONTOLOGIST 1902 901 



has a Hamilton expression, but each form bears a close resemb- 

 lance to an earlier representative of the genus. So striking is 

 the resemblance, that one, shown the magnified drawings, might 

 feel safe in pronouncing them to be from the fauna of the Onon- 

 daga limestone. Assigning these to their ancestral Hamilton 

 species has not been an altogether simple task, but has been car- 

 ried out by comparing the pyrite forms with the beaks of the 

 Hamilton species. These eight species of Spirifer, illustrate 

 better than any other group the results arising from a life in 

 an unfavorable medium. They are reversion types of the most 

 pronounced kind. 



Spirifer fimbriatus Conrad, mut. simplicissimus nov. 



Plate 3, fig. 1, 2 



The shell is almost circular in outline a trifle higher than broad. 

 It measures 1% mm in breadth, iy 2 mm in length and y 2 mm in 

 thickness. The ventral valve has a wide, moderately deep 

 median sinus and no lateral ridges. The dorsal valve has a short, 

 broad fold reaching about half way to the beak. The cast shows 

 the impress of a low, broad septum. The valves are only moder- 

 ately convex. This form is equivalent to the simplest form of 

 spirifer development, and comparable to S. fimbriatus 

 arrested in its earliest stage. It is not the young of some other 

 of the occurring species, as it is larger than S. fimbriatus 

 mut. pygmaeus, and far simpler than any other species of 

 its size. That S. fimbriatus should develop two mutations 

 is not remarkable, considering the variation the species shows 

 under normal circumstances. 



Localities. Canandaigua lake, Livonia salt shaft and Moscow, 

 in abundance. Total 52 specimens. 



Spirifer fimbriatus Conrad, mut. pygmaeus nov. 



Plate 2, fig. 8, 9 



The shell is almost circular in outline, being a trifle broader 

 than high. An average adult measures 1% mm broad, 1 mm 

 long and y 2 mm * n thickness. The ventral valve has a wide sinus 

 starting at the beak, and two low, wide folds on either side 

 reaching about half way to the beak. The beak is small and 



