52 A. E. Verrill — Study of- the family Pectinidoe. 



figs. 8, 9). Generally the inner surface is marked by ribs or flutings 

 corresponding to the larger external grooves, but the internal ribs 

 are usually bicarinate or double, especially near the margin, owing 

 to a special thickening along each of their margins, which renders 

 them more angular than the external gooves (pi. xvi, figs. 7, 10). 



External sculpture. — The surface may be smooth, or it may be 

 covered with sculpture of many kinds. It may differ on the two 

 valves, or not. The most common sculpture consists of numerous 

 strong radial ribs and grooves, alternating on the tAvo valves, so that 

 their ends interlock at the margin. The primary ribs as well as the 

 grooves may be covered with smaller ribs, or by scales or spines of 

 various kinds. The primary ribs may not increase in number with 

 age, but become continually broader [Pecten)^ or new ribs may be 

 introduced between them, so that the larger ribs become more and 

 more numerous without increasing in size [Chlamys). Concentric 

 sculpture maj^ be developed on one or both valves, either with or 

 without radial ribs. The whole surface may be evenly cancellated. 

 There may be regular concentric undulations {Hyalojyecten). The sur- 

 face may be covered with a fine, divergent, vermiculated structure 

 often described as "camptonectes sculpture" (pi. xvii, fig. 7, and 

 xviii, fig. 14a); this may coexist with radial ribs. 



Gills. — There are usually two pairs of normal tillibranchiate gills 

 (see pi. XX, fig. 6, g), but according to Dr. Dall, in at least one deep- 

 water species {Paramusium Palli), there is only a single pair of gills. 



Foot. — The foot shows considerable variation in form. It gener- 

 ally has a well-developed byssal groove, and usually a more or less 

 developed terminal slit, which is often so large that the end of the 

 foot can be expanded into a disk-like form (pi. xx, fig. 8). The 

 grooved side of the foot is turned obliquely downward to the right 

 (pi. XX, fig. 6,/"). The pedal retractor muscle is usually developed 

 only on the left side, so that its scar is lacking on the right valve. 



Palpi. — These are generally large and broadly triangular organs, 

 strongly fluted on the apposed surfaces. (PI. xxi, figs. 1, 2, (7, d! .) 



Labial tentacles. — These are usually much-branched organs sur- 

 rounding the mouth or situated at the sides and in front of it. They 

 may be few or many. Sometimes they are free, in other cases more 

 or less webbed together, and often more or less attached to the bases 

 of the palpi (pi. xx, fig. 6, and pi. xxi, figs. 1, la, 3, e). 



Sexes. — Some species are known to be dioecious, others are known 

 to be monoecious, but most of the genera and species have not been 

 studied with reference to this character. 



