NAPLES FAUNA IN WESTERN NEW YORK, PART 2 267 



Posidonia mesacostalis Williams (sp.) 



Plate 12, fig. 1-9 



Ptychopteria? mesacostalis Williams, U. S. Geol. Sur. Bui. 41. 1887. 

 P- 35. P 1 - 3, fig- 9 and ( ? ) var - fig- I2 



To bring this shell into comparison with species which have been 

 termed Posidonia, I have employed this name in preference to Ptychopteria. 

 Like the preceding species, it may have little in common with typical 

 Posidonia, perhaps as little with Ptychopteria, while one could find a justifi- 

 cation for assigning a shell with the peculiar expression of this to the genus 

 Leptodesma. The species, however, was another of the tenuous and 

 glabrous shells with little calcareous content in its constitution, and herein 

 differs from the Leptodesmas so abundant in the higher Devonic. 



Of 20 specimens of this shell before us, but few are right valves. The 

 original figures were of left valves only. 



Equivalve very oblique, linguate. Hinge line short and straight, 

 cardinal angles not extended. Beak at or behind the anterior third of the 

 hinge, anterior cardinal angle 90 , anterior margin vertical for a short 

 distance, thence bending abruptly backward with a broad inward curve, 

 basal margin subelliptic and almost wholly beyond the vertical dropped 

 from the posterior angle of the hinge line. Posterior margin subparallel to 

 the anterior, posterior cardinal angle obtuse. Beak projecting beyond the 

 hinge line. Umbo moderately convex, the convexity extending obliquely 

 backward. Surface marked by strong concentric festoons or undulations, 

 which are most widely separated along the median convexity of the shell. 

 In most of the best preserved specimens this is the chief character of the 

 surface, but some specimens show a fine radial marking of filiform, elevated 

 striae, which seem in part to pertain to the inner shell layer. These lines 

 are equally distributed over the surface and are not to be compared with the 

 median plications on P. a 1 1 i c a. Both valves of the shell have the same 

 form and surface characters. 



Dimensions of a large individual : length of hinge 6 mm ; total 

 oblique length 1 1 mm, hight 9 mm. 



