902 NEW YORK STATE MUSEUM 



acute. The dorsal valve has a short median fold and two lateral 

 folds, none of which reaches more than half way to the beak. The 

 septal impression is long and rather broad. Such a form resem- 

 bles the S. raricosta Hall (Onondaga), but it is not so gib 

 bous, nor are the folds so strong. It is a simplification of 

 S. fimbriatus, and about one twenty-fifth as large. The 

 dwarfing clearly began long before any shell was formed; for a 

 normal S. fimbriatus would not show a trace of a fold on 

 a shell 1 mm in diameter. 



Localities. Canandaigua lake, Livonia salt shaft and Moscow, 

 in abundance. Total 52 specimens. 



Spirifer mucronatus Conrad, mut. hecate Clarke 



Plate 3, fig. 13-15 

 Leiorhynchus ? heoate Clarke. U. S. Geol. Sur. Bui. 16. 1885. p. 31 

 This form is almost circular in outline, and not gibbous. It 

 measures 1% nim in hight, 1% nrm in breadth, and % mm i n 

 thickness. The ventral valve has a deep median sinus, and on 

 either side five ridges, which reach about two thirds of the 

 way to the beak. The median fold of the dorsal valve reaches 

 a trifle over halfway to the beak, as do also the five lateral 

 folds on each side. Near the blunt beak, the septum is deeply 

 impressed. Thence a shallow groove runs along the middle 

 of the median fold to the margin. The beaks are close, the 

 ventral one overhanging the dorsal. Such a form suggests 

 S. gregarius (Onondaga) in its younger forms. The re- 

 semblance to the adult S. mucronatus is not striking, 

 but the groove in the fold of the dorsal shell, the low area, and 

 the similarity to the umbo of S. mucronatus, indicate 

 that this mutation is a modification of the above species. 



Localities. This is the commonest of all the occurring forms 

 and is found at Canandaigua lake, Livonia salt shaft and 

 Moscow. Total specimens 116. 



Spirifer medialis Hall, mut. pygmaeus nov. 



Plate 3, fig. 9, 10 



This form is broadly oval in outline, a trifle broader than 

 high. It measures 2% mm in width, 2% mm in hight and iy 2 



