4 8 



NEW YORK STATE MUSEUM 



Mytilarca acutirostrum Hall 



Plate 5, fig. ii, 12 



Ambonychia acutirostra Hall. N. Y. State Cab. Nat. Hist, noth An. Rep't. 



1867. p. 336, pi. 14, fig. 2 

 Ambonychia acutirostra Hall. N. Y. State Mus. 28th An. Rep't. 1879. 



p. 171, pi. 7, fig. 12 



Several internal casts of this shell, rather below medium size, were 

 obtained from the lower bed at Shelby. They are characterized by their 

 full, relatively long valves with produced, acute beaks, straight, short car- 

 dinal line, straight and slightly convex anterior margin which extends 

 almost the whole length of the valve, very convex basal and less rounded, 

 almost straight and oblique posterior margin, which forms an obtuse angle 

 with the short wing of the posterior cardinal region. The umbo is very 

 prominent and convex ; from it a slightly elevated and distinct umbonal 

 ridge extends to the anterobasal angle. Thence the valve slopes evenly 

 toward the posterior margin, and more abruptly toward the anterior margin. 

 No muscular impression has been observed on the somewhat incrusted casts. 

 The impression of a distinct, narrow ligamental area, extending the full 

 length of the cardinal line, and of two oblique lateral teeth at the extreme 

 posterior end of the cardinal line are noticeable. 



Observations. This form differs from the foregoing both in size, and 

 in its slightly convex anterior margin. We have identified these shells 

 with Ambonychia acutirostrum, a species which was described by 

 Hall from the limestone of the age of the Niagaran group, near Mil- 

 waukee. In the 28th report of the New York State Museum it is said to 

 be associated, in Wisconsin and Illinois, with Ambonychia aphaea, 

 described from Wauwatosa Wis. and Bridgeport 111. Professor Whitfield 1 

 cites the species from the Racine beds of Racine, Greenfield, Waukesha 

 and Wauwatosa, and from the Guelph beds of Cedarburg, it being the only 

 Guelph lamellibranch mentioned besides Megalomus canadensis. 

 It has not been reported from the Canadian Guelph. 



'Geol. Wisconsin, 2:372-79. 



