332 Thirty-fifth Eeport on the State Museum. 



for a more important organic marking. Partially exfoliated shells 

 show some obscure radiating lines which belong to the intimate shell- 

 structure, and are not external. 



Posterior muscular impression situated near the middle of the 

 posterior slope, and marked by irregular vascular striae ; from the 

 lower anterior side of this impression the pallial line extends slightly 

 downward, curving forward, and continuing in a line nearly parallel 

 to the margin it terminates in a small muscular impression within 

 the rostral cavity. The pallial line is frequently pitted or inter- 

 rupted in its course, and from these pits, radiating grooves extend 

 toward the margin of the shell. Ligamental area extending the 

 entire length of the hinge, comparatively wide, grooved longitudi- 

 nally with fine, continuous parallel lines. Hinge with one or two 

 linear, slightly oblique, lateral teeth on the posterior side, and with 

 numerous erect or oblique irregular folds along the cardinal line be- 

 low the ligamental area, which terminate on the inner margin in 

 distinct crenulations. The cardinal wrinkles are smaller and more 

 regular anteriorly, becoming large and irregular posterior to the 

 beak. 



Three large specimens have the following respective dimensions : 

 length 90, 70 and 81 mm., height 100, 99 and 81 mm., hinge-line 

 100, 90 and 90 mm. An example of medium size has a length of 

 55 mm., height 76 mm., and hinge-line 48 mm. A large, erect in- 

 dividual measures 72 mm. in length, and 90 mm. in height. A 

 small right valve has a length of 23 mm., height 27 mm., and hinge- 

 line 33 mm. 



• 

 This is an abundant species and attains a very large size in older 

 individuals. In its different conditions of growth and preservation, 

 it presents a very ^reat variety of aspect, and it becomes extremely 

 difficult to determine the limits of the species. In the young shell 

 the posterior wing is proportionally more extended than in older in- 

 dividuals, while the anterior wing is usually smaller. During the prog- 

 ress of growth, both wings often become extravagantly developed, and 

 the form and proportions of these parts cannot be relied upon for 

 specific distinctions. The characteristic forms have the body nearly 

 erect, with the umbo and beak directed slightly forward ; but associ- 

 ated with these are other forms which present a considerable degree of 

 obliquity in the body of the shell, and while there seem to be no con- 

 stant marks of specific distinction, it is extremely unsatisfactory to 

 group them all under one species. 



The interior characters also present considerable differences, the 

 number of lateral teeth varying from one to three ; the cardinal 



