34 GENERA OF SHELLS. 



not very distant, yet are always separated by a narrow, 

 furrowed, rather hollow facet, to which the ligament is 

 attached, and which distinguishes them from the nu- 

 culse. The shells are all marine, and resemble the 

 pectines by their form, and by the internal margin 

 being always crenate. Many species acquire consid- 

 erable thickness by age, and such a change of form as 

 renders it difficult to distinguish them. 



(a.) Distant, longitudinal furrows, and frequently fine trans- 

 verse or longitudinal stria. 



P. glycimeris, pilosus, undulatus, marmoratus, scriptus, 

 pennaceus, rubens, angulatus, stellatus, pallens, violaces- 

 cens, zonalis, striatularis, nummarius. 



(b.) Prominent and radiating longitudinal ribs, with or with- 

 out transverse strice. 

 P. castaneus, pectiniformis, pectinatus, radians, vitreus. 



NUCULA. Shell transverse, ovate triangular, 

 or oblong, equivalve, inequilateral ; no area 

 between the beaks. Hinge linear, broken, 

 many toothed, interrupted in the middle by 

 an oblique, spoon-shaped pit ; teeth numer- 

 ous, subacute, often produced as in the pec- 

 ten ; beaks contiguous, curved backwards ; 

 ligament marginal, partly inferior, inserted 

 into the cardinal fossa, or pit. 



The nuculus is distinguished from the pectunculus 

 and area, not only by the broken, angular line of their 

 hinge, but also by the ligament, which is partly inter- 

 nal, and by wanting the facet between the beaks. They 

 are small sea shells, somewhat triangular, and more or 

 less pearly on the inside, 



N. lanceolata. rostrata, pella, Nicobarica, obliqua, marga- 

 jitacea limatula, proxima, nucleus, recurva. 



