LAMELLIBRANCHIATA. 115 



an extension of the shell along the post-basal margin which is marked by 

 concentric striae and undnlations, while the rays l)ec()me interrupted and obsolete. 



After a study and comparison of a very large number of individuals, it has 

 not been fovmd jJossible to draw any satisfactory line of specific distinction. 

 The figures on plate xix illustrate all the important varieties of form that have 

 been observed. 



The specimens illustrated in figures 2, -I, 4 of plate xix, represent the typical 

 form of A. quadrula, Conrad; the figures 9, 10, 11 of the same plate more 

 nearly correspond to the A. Boydii, Conrad. All the right valves show the same 

 degree of convexity and the same surface characters, and vary in form no more 

 than the variations i]i the left \alve. The extreme forms seen in figures 2() and 

 27 are old shells, showing the irregular growth of the margins, leaving the upper 

 portions of the shell of normal form and surface characters. 

 The following are Mr. Conrad's descriptions: 



" Avicula Boydii, pi. 12, fig. 4. — Su))-rhond)oidal, compressed, lower valve 

 " with numerous radii, disposed to be interrupted by concentric lines, which 

 " are fimbriated, or have numerous angular indentations; anterior wing short, 

 " sinuous, truncated; |)osterior wing ample; postei'ior exti'cmity of the valves 

 " acutely rounded." 



"Avicula quadrula, pi. 13, fig. 5. — 8u))-(pia(li-atc' ; length and width equal, 

 " compressed ; lower valve piano-con \ex, with distant radii of equal size, 

 " crossed by concentric lines ; posterior wing rather shorter than the width of 

 " the shell ; anterior wing triangular, beneath which the margin is direct before 

 " rounding to the beak." 



The unrecognized form is indicated as follows : 



" Avicula pleuroptera, pi. 13, fig. 2. — Lower valve sub-ovate, ventricose, with 

 " obsolete radii ; summit of umbo above the cardinal line ; anterior wing very 

 " short ; the posterior wing produced, rostrated, acute, with an oblique rib or 

 " carina on tlie posterior half, extending to the acutely angulated extremity." 

 The oblique rib or carina mentioned was pioliably one t)f the lateral teeth as 

 shown in figure 12, plate xix. 



The Avicula angustirostra of Conrad, cited as occuiTJug at Middleville, Schoharie 



