[WILLIAMS] FOSSIL FAUNAS OF THE ST. HELEN'S BRECCIAS 221 



as no representative of this section of the gemis has been recorded higher 

 than the upper Oriskany. The only representative of the genus of later 

 (late, Eaionia? varkibilisr -" Whiteaves, if an Eatonia, belongs to the 

 broadly plicated section including Eaionia medialis. 



15. Spirifer arenosus (Conrad). 



Spirifer arenosus is represented in this fauna by several well marked 

 specimens. Mr. Schuchert, in 1901, expressed doubt as to the occur- 

 rence of this species in the St. IJelen's fauna.^ It does not occur in the 

 fauna of lots 2 or 3, which are evidently of lower position than 252.1, 

 but there is no doubt as to the specimens here listed ; the characteristics 

 of the species are well expressed on several typical specimens, one of 

 which is 55 mm. broad and with well developed plications in the sinu:s 

 of the pedicle valve. 



The variants of Spirifer concimim, refeiTed to by Schuchert, with 

 plications in the sinus are smooth at the initial stage in both the fold 

 and sinus, and when the rudimentary plications do appear they are al- 

 ways inferior in size and more angular than the normal plications of 

 the lateral surface. In the specimens before me they are of equal 

 strength and of rounded form similar to the adjacent plications ©ut- 

 side the fold or sinus. Also, whereas in *?. cojidnniis the sinus of the 

 pedicle valve is sharply defined by the strong bounding plications on 

 each side, the St. Helen's form has the sinus broad, sloping gradually 

 into the sides, and the boundary between them is indistinct as is the case 

 with typical Spirifer arenosus. From an evolutional point of view the 

 St. Helen's and Grande Grève specimens, with few plications at the 

 initial stage of development of the sinus, indicate an early rather than 

 late stage of evolution. But the fact that specimens of both this and the 

 fullv developed type occur together in the Grande Grève fauna indicates 

 that the full evolution of these characteristics of the species were rapidly 

 accomplished and at or near to the first appearance of the new type. 



The median plication in the sinus of the pedicle valve is distinct 

 near the beak and remains prominent except in the larger specimen in 

 which it divides into two or sometimes three plications before reaching 

 the front. At a distance of 30 mm. from the beak in one specimen only 

 three plications are visible, but they are of the rounded form character- 

 istic of Sp. arenosus. 



Well developed representatives of the species are present in the 

 Moose Eiver sandstone of Maine, showing a similar stage of development 



1 Contr. Canada Paleontology. 1. 1891, p. 233, PI. 29. Figs. 6-9. 



2 Am. Geologist, XXVII. p. 249. 



