A. E. Verrill — Study of the family Pectinidce. 51 



become neai'ly transverse and tooth-like, alternating with pits {Pal- 

 lium) . 



The number of cardinal ribs may be from one to three on each 

 end of the hinge-plate ; most f requentl}^ there are one or two, the 

 upper one forming the inner boundary of the ligamental groove, the 

 second one somewhat divergent from the first. (PI. xvi, fig. 6, t.) 



Cross lines or incisions. — The upper ribs, and sometimes the other 

 cardinal ribs, are generally crossed by numerous fine transverse 

 grooves or incisions, alternating with ridges of about the same 

 width. These cross-lines may be straight and regular or they may 

 be crooked or vermiculate. They are generally more distinct in the 

 young shell than in the adult (pi. xvi, figs. 6, 9, 12 a, i ; pi. xix, figs. 

 1, 2, 7). In certain extinct early genera they were much more con- 

 spicuous than in any existing forms [Neithea, Grenipeoten). 



Auricular cruras. — These are divergent raised ribs or faint ridges 

 running along the inner margins of the auricles (pi. xix, figs. 6-9). 

 Sometimes they terminate distally in a rounded denticle (pi. xvi, fig. 

 9, d) ; sometimes the denticle alone is present. They are often obso- 

 lete in the thick-shelled species, but are sometimes conspicuous struc- 

 tures {Am,usium). 



Pesilinm. — This is generally nearly central and triangular or 

 wedge-shaped. The surfaces next the shell are often calcified. 



Pesilial pit or Chondrophore. — This may be excavated entirely 

 within the outline of the hinge-plate, or it may project considerably 

 below it. In typical Pecten the pits are unlike in the two valves. 

 (PL xvi, figs. 6, 9, r ; pi. xxi, figs. 2, 2a, r.) 



Ligamental groove. — This is always narrow, submarginal, and ex- 

 tends along the whole length of the hinge-line. The elongation of 

 the auricles serves to give it greater extension and importance; the 

 exterior margin of one valve is often curved inward, partly over the 

 groove. (PL xvi, figs. 6, 9, I ; pL xxi, fig. 2, /.) 



Muscular scars. — The scars left by the adductor and pedal muscles 

 often show marked differences that are, perhaps, of generic value, 

 but unfortunately they are often very indistinct, and in the smaller 

 species nearlj^- or quite invisible. The scars differ considerably in 

 the two valves, for the pedal retractors are lacking ' in the right 

 valve. The pallial line is very simple. (PL xxi, figs. 2, 2a.) 



Internal ribs and lirm. — The inner surface of the shell may be per- 

 fectly smooth in some of the small plain species, but in Amusium 

 and some other groups special raised radial ribs, often opaque white in 

 color, are developed independently of any external sculpture (pi. xix. 



