GUELPH FAUNA IN THE STATE OF NEW YORK 49 



Hall referred his species with doubt to Ambonychia. Ulrich 1 refers 

 the form to Mytilarca, a view which is verified by the evidence of lateral 

 teeth. 



Mytilarca acutirostrum is, as indicated by Hall, very similar 

 to Myalina mytiliformis Hall from the gray Clinton limestone of 

 New York. From the latter species M. acutirostrum was said to differ 

 in its more acute beak and relatively greater width. In regard to width 

 our specimens are intermediate between the two species, but in the charac- 

 ter of its beak it is more like the western Guelph form. Foerste 2 has 

 described still another Clinton form as Mytilarca mytiliformis, 

 but as Hall's Ambonychia mytiliformis is a Mytilarca [p. 560 of 

 same paper] the species should be renamed (M y t. foerstei nom. 

 propos.}. 



ptbbinba Goldfuss. 1826 



Pterinea subplana Hall (sp.) 



Plate s, fig. 4 



Avicula subplana Hall, Paleontology of New York. 1852. 2:283, pi. 59, fig. 

 3^, 3b, 3 C 



Internal casts from the Rochester chert and the upper horizon at 

 Shelby indicate a depressed right valve with long straight hinge line, beak 

 subanterior and not prominent, slightly projecting above the hinge line. 

 The ear is short and apparently rounded, the posterior wing is much 

 extended, obliquely truncated and set off from the body of the valve by a 

 low and broad depression. A long narrow cartilage pit extends from the 

 beak three fourths the length of the posterior cardinal line and parallel to 

 it. Surface with concentric lines. 



The left valve shows a greater convexity and similar outline. 



Avicula subplana is a Rochester shale species which has not been 

 reported from the Guelph of Canada or the Interior. 



"Geol. Sur. Minnesota, v. 3, pt 2, p. 494. 

 *Geol. Ohio, 7:559. 



