188 Systematic Paleontology — Middle Devonian 



vex; mesial fold prominent, flat or rounded above and frequently marked 

 by a median groove. Beak incurved ; cardinal area extremely narrow, 

 about one-third as high as that of the ventral valve. Surface marked by 

 from eight to fifteen or more subangular plications on either side of the 

 mesial sinus or fold, the outer ones of which do not reach the beak. On 

 well preserved specimens the plications are crossed by numerous fine 

 lamellose striae which become crowded and closely imbricating towards 

 the front of the shell, sometimes marking several lines of interrupted 

 groAvth. 



At some localities in Maryland thi?i species is very abundant forming 

 a considerable part of the rock. As illustrations may be cited the locality 

 about two miles north of Hancock on the road from that town to Har- 

 risonville, Pa. ; one near tlie bridge over Town Creek three miles northeast 

 of Oldtown; while on Evitts Creek below Wolfe Mill are specimens with 

 the shell beautifully preserved. Among the Maryland specimens may be 

 found examples of most of the forms figured in Palaeontology of N. Y. 

 (see vol. iv, pi. xxxiv). Figures 10 and 11 of this report repre^^ent a 

 specimen from Evitts Creek with truncate cardinal angles similar to the 

 narrow form shown by figure 8 in the Palaeontology of New York ; while 

 at McCoys Ferry an extremely miicronate specimen was obtained with a 

 length of In mm., and a width of 3.5 mm. for one-half of the shell along 

 the hinge-line, of which the mucronate point was fully 20 mm. in length. 

 The great majority of the specimens are exfoliated and furnish only 

 internal impressions. 



Length of nonmucronate specimen, 15 mm.; width, 26 mm.; length of 

 nonmucronate triangular shaped specimen, 22 mm. ; width, 45 mm. ; 

 length of average specimen, 16 mm. : width, 39 mm.; length of mucronate 

 specimen but not extreme, 15 mm.; width, 60 mm.; length of another 

 specimen, 18 mm. ; width, 50 mm. 



In reference to the synonymy of this species Miller and Schuchert con- 

 sider Delthyris mucronata Con. as a synonym of Terehratula pennata 

 Atwater; while Dr. J. M. Clarke states that "I see no good reason for 

 calling Spirifer mvcronatvs Conrad S. pennatiis Atwater. It may be true 

 that the latter is what we mean bv 8. mucronatus but it would be difficult 



