3<D NEW YORK STATE MUSEUM 



the valves abrupt. Beak subterminal, elevated, umbonal ridge subparallel 

 with the anterior margin. From this ridge the surface of the left valve 

 slopes very gradually downward and back ; right valve flat except at the 

 beak. Surface of left valve bearing sharp and numerous radial lines, 

 crowded and with a tendency to fasciculation over the anterior and lower 

 parts but equidistant posteriorly. The hinge and ligament areas are 

 bounded and crossed by a few very strong striae, the cardinal edge of the 

 valve being thickened. All these lines are crossed by concentric striae, 

 short and elevated everywhere except on the posterior cardinal surface. On 

 the right valve the radial lines are obsolete except on the posterior wing 

 near the hinge, only the concentric lines standing out sharply and equi- 

 distant. The inner surface of both valves is quite smooth. This is a strik- 

 ing species well denned by its outline and surface characters. Its thin shell 

 has left insufficient evidence of its dentition but I have referred it to Ptero- 

 nitella largely because of its general aspect. 

 Horizon. No. 1 1 . 



Pteronitella passer Clarke 



Plate 4, figures 12-14 



Pteronitella passer Clarke. N. Y. State Mus. Bui. 107. 1907. p. 212 



This differs from the preceding in presenting a less extended and rather 

 blunt posterior extremity, a more conspicuous anterior ear and a relatively 

 greater breadth. The outline is still elongate with a gentle surface slope 

 on all sides except the front where it is quite abrupt. The surface is fully 

 reticulated by radial and concentric lines, the former being as before stronger 

 along the posterior wing. 



In my judgment this will be readily distinguished by its outline as 

 exhibited in two left valves here figured, though it is undeniably similar to 

 P. h i r u n d o in many of its characters. 



Horizon. No. 1 1. 



Mytilarca dalhousie Clarke 



Plate 5, figures 18-22 



Mytilarca dalhousie Clarke. N. Y. State Mus. Bui. 107. 1907. p. 216 



Cf. M. ovata Hall. Palaeontology of New York. 1859. 3:279, pi. 50, fig. 7. 



M. soli da Maurer. Fauna d. rechtsrhein. Unterdevon. 1886. p. 13; T'rech. 



Devon. Aviculiden Deutschlands, p. 143, fig. 15 



Mytilarca is not common in early Devonic faunas. The specimens of 

 the genus from the Dalhousie fauna are well developed in respect to generic 

 characters and of moderately large size approaching in dimensions the Hel- 

 derbergian species Megambonia ovata Hall which has never been 

 well described or figured though its relation to Mytilarca has long been 

 recognized; 1 and in outline and contour Myalina soli-da Maurer of 



1 N. Y. State Mus. Mem. 3. 1900. p. 89. 



