[WILLIAMS^] FOSSIL FAUNAS OF THE ST. HELEN'S BRECCIAS 229 



to be " well expressed through early stages and till the convexity is 



attained" " thence forward this character becomes modified." . . . . 



(" &?/ the rapid increase in number of the larger strice and diminution 

 in size and number of the intervening striœ.") But the author further 

 states that there may be *' large sJiells with adult convexity retaining the 

 fasciculate striation of early stages." In ou,r specimens such large speci- 

 mens do retain the fasciculate striation of early stages, which gives them 

 the cliaracters of the typical 6'. arœta. Hall. 



11. Stropheodonta planulata. Hall. 



11a. Stropheodonta Blainvillei, Billings. 



lib. Stropheodonta per plana, Hall. 



Several specimens of the flat finely striate form of Stropheodonta 

 are in the collection. 'The smaller forms 25 to 30 mm. wide agree well 

 with Hall's species 8. planulata; others attaining a width of 45 mm. ai^» 

 not disting-uishable from Billings's species ;S'. blainvillei, nor do either 

 present in the condition of preservation (the interior muscular markings 

 are not exhibited) any substantial characters by which they can be dif- 

 ferentiated from the Middle Den^onian species, S. perplana. The three 

 specimens may be distinct or they may belong to a single species, but for 

 purpose of correlation the citation of either name cannot be regarded as 

 representing any particular horizon, though differences in size and thick- 

 ness of the shell may be associated with particular environmental condi- 

 tions of growth. 



12. Stropheodonta Bechi, Hall. 



This 'species is represented by several specimens, the largest showing 

 a width of 43 mm. 



13. Strophonella punctulifera, Conrad. 



14. Strophonella (Amphistrophia) contlnens^ Clarke. 



15. Strophonella cavumbona^ Hall. 



As was notefl by Hall this species presents considerable reseinhlance 

 in form to the common Silurian form of Europe, S. eaglyphra. In liris 

 description of Conrad's species Hall noted the punctured surface, but did 

 not observe the fascicular arrangement of the surface striae. There are 

 among St. Helen's specimens identified with S. punctulifera both the 

 punctate inner layers and the fasciculate external surface lineations. 

 AVhen partially exfoliated the peculiar panctations are conspicuous. Tlie 

 combination of the two characters was noted by Billings on one speci- 

 men 1 from the Gaspé limestone vv^hich he described as a variety of 

 ^S". punctulifera. Clarke,^ in illustrating the 'species, recognizes this 

 variety under the name Strophonella {Amphistrophia) continens. 



1 Billings, Paleozoic Fossils, Vol. II, Pt. 1, 1S74, p. 32. 



2 Clarke, p. 195, PI. 40. 



