A. E. Verrill — Study of the family Pectinidm. 67 



These may be regarded as primitive and simple forms, from which 

 larger species with stronger ribs have been developed without losing 

 this primitive fine sculpture. But this kind of sculpture depends 

 upon structural features that must, be taken into account in the 

 classification of the family. Possibly all true species of Pseudamu- 

 skcm have, also, the camptonectes sculpture, but I have not at hand 

 sufficient material to settle this question. 



It appears to be lacking in Gyclopecten, Pectinella (typical), Prop- 

 eamiisiuni, Amusium^ Paramusimn, Pecten^ Lyropecten, and Pcd- 

 lium. Nor have I observed it in various species of ^quipecten 

 examined. It is also absent from many species of Ghlamys (restr.) 



.SJquipecten Fischer, Manual, 1887. Type, Chlamys opercularis (L.) 

 Plate XVI. figs. 6-11. Plate XX. figs. 1-3, 6, 6a. 



Shell broadly rounded, with the valves nearly equal and sym- 

 metrical. Auricles well formed, angular ; byssal notch well-devel- 

 oped. The sculpture consists of a moderate number of large and 

 nearly equal primary radial ribs, which increase in size, but are not 

 much increased in number with age, by the interpolation of new ones. 

 Internal ribs or flutings correspond to external grooves, but each one 

 is bicarinate or double, especially near the margins. Hinge-plate 

 with one or two slightly divergent ribs at each end, often crossed by 

 strong transverse incisions. Pectinidial teeth abortive in the type, 

 but present in most species. Foot of the type-species is subcylin- 

 drical, well-developed, with a byssal fissure and a terminal deeply 

 bilobed "scoop-shaped" disk, which can be expanded. In our 0. 

 irradians (see pi. xx, fig. 6) the foot has a similar structure, but the 

 terminal disk appears to be smaller. In the type there are (t. Jef- 

 freys) 35 to 40 ocelli, and two or three irregular rows of tentacles. 



The type-species of this group ( G. opercularis) has a well-rounded, 

 scarcely oblique shell, with the valves a little unequal and conspicu- 

 ously gaping at both ends, but especially so anteriorly. The auricles 

 are well-developed, angular at both ends, and nearly equal. The 

 byssal notch is large, but the pectinidial teeth are nearly or quite 

 obsolete in the adult. Both valves have about twenty-one large 

 primary ribs, which are at first narrow and angular, but become 

 broad and rounded by growth. In the left valve both ribs and 

 grooves are covered with small radial riblets, decussated by fine 

 concentric laraellaj. In the right valve the radial riblets are less 

 distinct, but evident. The interior has about twenty-one bicarinate 

 ribs or flutings on each valve. The hinge-plate has one broad, low. 



