EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 3 I 



the Lower Coblentzian where such species are rare. Mytilarca dal- 

 h o u s i e is elongate subovate in outline, with short straight posterior hinge 

 and long abruptly deflected anterior margin extending to the basal curvature 

 of the valves with a slightly sinuous curve. The surface is regularly but 

 slightly convex with the greatest elevation along the anterior crescence line 

 and the slope thence gradual in all directions except anteriorly, where it is 

 curved down and inward. The general expression of the shell will be 

 better appreciated from the figures than by description. There are large 

 and small shells present with these characters, all representing the same 

 specific form. 



The hinge characters are excellently shown in one specimen of the left 

 valve. The beak is terminal ; beneath it is the apex of the broad ligament 

 area which is strongly striated horizontally. The anterior edge of this 

 area slopes obliquely back to the inner apex of the valve and at this point 

 is a single oblique elongated tooth, doubly crenulated on the crest leaving 

 between it and the edge of the ligament area a pit or socket for the recep- 

 tion of the tooth of the other valve. This socket is bounded below by a 

 continuation of the tooth. Toward the posterior end of the hinge is an 

 oblique but obscure ridge below the ligament area. This hinge structure 

 is in agreement with Hall's delineation of it for the genus Mytilarca. The 

 umbonal region of the shell is thick and the posterior margin shows 

 successive thickened layers of shell growth. 



Horizon. Nos. 9, 10, 11. 



Modiomorpha impar Clarke 



Plate 6, figures 6-8 



Mo dio m orph a i m p ar Clarke. N. Y. State Mus. Bui. 107. 1907. p. 217 



Shell of small or medium size with straight hinge line not extending' 

 for the full length of the valves. Beaks anterior but not terminal, depressed, 

 the umbonal region rising gradually and soon broadening out over the low 

 posterior slope. In front of this ridge the surface is gently depressed mak- 

 ing a distinct sinus in the lower margins specially on the left valve. Ante- 

 rior margins relatively narrow and blunt, posterior extremity broadly 

 rounded. Postumbonal slope gently concave. Surface covered with 

 regular concentric lines. 



Horizon. Nos. 9, 10, 11. 



Goniophora curvata Clarke 



Plate 4, figures 22, 23 



Goniophora curvata Clarke. N. Y. State Mus. Bui. 107. 1907. p. 225 



Shell of medium size, elongate, hinge line usually concealed, but appa- 

 rently short, not extending posteriorly to within one third of the shell's 



