NO. 1985. NEW SPECIES OF SILURIAN FOSSILS— WILLIAMS. 321 



Our species is, however, less globose and smaller than Sowerby's 

 figure of Atrypa didyma. The surface is smooth except for con- 

 centric growth lines on the outer half of the shell. The interior of 

 the pedicle valve (see fig. 4) shows the disposition of the dental 

 plates and muscular scars as represented by Davidson's ^ figure of the 

 enlarged interior of a pedicle valve of "MeristeUa didyma" which 

 resembles Hyattella and Meristina more than Whitfieldella Hall. 

 The scar on the brachial valve is linear and there is a hinge plate; 

 whether cleft or not is not evident, but there is no medium septum. 

 The spiral ribbons are arranged in two cones opposed as in Merista 

 and Hyattella. There appear to be at least 8, perhaps 9, coils each 

 side. None of the specimens show with distinctness the jugum 

 and central lamellae, but the parts preserved are well represented 

 by the figures given by Davidson of the spirals of "Ileristina 

 didyma." ^ 



From these observations it is evident that our species represents 

 the form originally described by Sowerby under the name Atrypa 

 didyma Dalman. 



From the fact that it does not have the prominent beak of the 

 Gotland specimens,' a form which was included under the same 

 name by Davidson^ it does not seem clear that the British form 

 figured by Sowerby is specifically the same with Terhratula? didyma 

 Dalman. It is also uncertain what may be the interior characters 

 of Sowerby's original specimens. Under these circumstances a new 

 specific name is appropriate, although there are not in evidence 

 characters by which the Maine species may be specifically distin- 

 guished from the form described and figured by Sowerby. The 

 specific name WhitjieldeUa edmundsi is therefore proposed for this 

 species. 



Generically it is difficult to assign the species strictly to any one 

 of the genera in use. The absence of a medium septum from the 

 brachial valve technically excludes it from WJiitfieldella which was 

 erected with the species Atrypa nitida Hall as type. Meristina Hall 

 and Whitfieldia Davidson also are defined as having a median sep- 

 tum in the brachial valve. Hyattella Hall, closely related to Whit- 

 jieldeUa Hall, has no median septum. In this dilemma and until 

 fuller knowledge of the characters are ascertained I will provisionally 

 refer the species to Hall's genus WhitjieldeUa, recognizing Davidson's 

 reference of Sowerby' species to Meristina and using the term WTiit- 

 fieldella in the larger sense as the name "proposed as a substitute 

 for the term Meristina in its current application to species not con- 

 generic with M. maria." ^ 



1 Sil. Brach., pi. 12, fig. 9. 



s Brit. Pal. Brach., suppl., pi. 4, figs. 23 and 23o. 



» Hall and Clarke Pal. New York, vol. 8, pt. 2, pi. 11, figs. 14, 15. 



* Brit. Sil. Brach., pi. 12, figs. 4, 8. 



6 Pal. New York, vol. 8, pt. 2, p. 60. 



80459°— Proc.N.M.vol.45— 13 21 



