50 A. E. Verrill — Study of the family PectinidcB. 



In many respects, Amusmm seems to be one of the most special- 

 ized groups. It may have descended from ribbed species of earlier 

 ages. When adult the byssal oi-gans are obsolete and the highly 

 modified foot, with its large, terminal, concave disk, serves as a 

 pushing organ, and perhaps for creeping about. The auricles and 

 byssal notch are degenerate in form, while the internal strengthening 

 ribs or lirae are peculiar, secondarily acquired features, due to the 

 special development of the thin shell for active swimming through 

 life. 



Synopsis of the principal characters available for the classification 



of Pectinidoi. 



Shell. — The form may be broadly rounded, or high and narrow ; 

 oblique, or upright ; the texture may be thin and hyaline, or thick 

 and opaque. Sometimes a somewhat pi-ismatic or partly pearly 

 structure appears on the inside, but the inner surface is generally 

 porcellaneous.% 



The valves may be nearly equal, or one valve (either right or left) 

 may be less convex, or even flat ; they may gape widely at both 

 ends, or close almost completely ; the margin may be simple and thin 

 and meet evenly, or the edge of the lower valve may bend up against 

 the upper, or it may be bevelled ; more frequently the margins are 

 scalloped and interlock. In some cases (Hinnites) the right valve 

 becomes cemented to foreign objects and irregular in form, when 

 adult. 



Auricles. — These may be small or large; straight or oblique; 

 prominent and angular, or poorly developed and obtuse ; equal or 

 unequal. The ends gape or flare apart, more or less, to contain the 

 pallial tentacles and ocelli. The posterior auricles are sometimes 

 nearly obsolete. 



Byssal notch. — This may be deep, or shallow, or even obsolete (as 

 in Ariiusium and Pallium). As its margin grows it often leaves a 

 " f asciole," indicated by lines of growth, behind it. 



Pectinidial teeth. — These may be strong or feeble ; few or many ; 

 sometimes they are absent, especially in adult shells of large size. 

 (PI. xvi, fig. 9,^.) 



Hinge-plate. — This may be thick or thin ; broad or narrow ; plain, 

 or bearing longitudinal ribs, or oblique teeth-like processes (pi. xxi, 

 fig. 4). 



Cardinal ribs. — These are longitudinal ribs or folds, either nearly 

 parallel with the hinge-margin or somewhat oblique ; or the}'- may 



