40 ha.ma.rck's Geneva of Shells. 



4. Pecten*. 



Shell free, regular inequivalve, auriculated ; inferior margin 

 transverse, straight; beaks contiguous, with no intermediate 

 facet. Hinge without teeth : a triangular cardinal pit, wholly 

 internal, receiving the ligament. 



The pectines are almoSt always longitudinally radiated with 

 fine or coarse ribs ; the base of the shell is terminated by a 

 straight, transverse line, beyond which the beaks never project. 

 The valves are generally thin, of equal size, but not equally 

 convex, the upper being almost constantly flattened ; their tex- 

 ture is not loose-foliated like that of the ostrese. They are sea- 

 shells, much diversified, very numerous in species, and the spe- 

 cies not easily determined ; they are usually ornamented with 

 various and brilliant colours. They are always auriculated, and 

 the largest ear is on the posterior side, and beneath it is a sinus. 



The species are subdivided into (1) Shells with the ears 

 equal, or nearly equal, — 26 species ; and (2) Those with the 

 ears unequal, — 32 species. 



Type. Pecten maximiis t, (Ostrea maxima. Linn.) 



Shell inequivalve, upper valve almost flat ; radii rounded, 

 longitudinally striated. European Seas. In all 59 recent spe- 

 cies, and 26 fossil. PI. II. Fig. 89. 



5. PlicatulaJ. 



Shell inequivalve, not auriculated, contracted towards the 

 base ; upper margin rounded, subplicate ; beaks unequal, with 

 no external facet. Two strong cardinal teeth on each valve, 

 with an intermediate pit which receives the ligament ; ligament 

 wholly internal. 



The plicatulse are sea-shells ; they differ from the pectines by 

 having cardinal teeth, and being without ears; and from thespon- 

 dyli, by having no external facet, nor consequently the inter- 

 mediate furrow, occasioned by the ligament of the spondyli ; 

 nor are they spinous, like those shells. 



• A comb. Also the originalJ^Latin name for] all shell-fish, striated, or 

 ribbed like cockles." 

 ^ The Uirgest t Dim. from plica, a, fold, or imnkk. 



