THIGONIA. CONCHIFERA. 91 



Gen. 9. TRIGONIA, Lam. 



Shell equivalve, inequilateral, trigonal, sometimes orbicular ; 

 primary teeth oblong, flattened on the sides, diverging, fur- 

 rowed transversely ; two in the right valve grooved on each 

 side, and four in the other valve grooved on one side ; liga- 

 ment exterior, marginal. 



The shells of this genus, except one species, are only known in the fossil state. 

 In Britain the fossil species occur in the green-sand and oolite. 



T. pectinata, Lam. Shell stiborbicular, with radiated ribs ; ribs ele- 

 vated, verrucose ; margin plicated ; inside pearly. If inch broad. 

 Inhabits seas of New Holland. An. Mus. iv. pi. 67, fig. 2. 

 This is the only recent species known. It resembles a Pecten without ears. 



FAMILY III. ARCACEA. 



Primary teeth small, numerous, and disposed in a line on each 

 valve, either straight or interrupted. 



Gen. 10. NUCULA, Lam. Area, Lin. 



Shell oval-trigonal, or oblong, equivalve, inequilateral ; hinge 

 linear, many toothed, interrupted in the middle by an ob- 

 lique hollow ; teeth pectinated ; beaks contiguous, bent back- 

 wards ; ligament marginal, and partly internal. 



The shells of this genus are small, marine, of a trigonal form, more or less nacre- 

 ous internally. They are found both in a recent and fossil state. The fossil species, 

 however, are most numerous, and in Britain have been found in London clay, 

 chalk marl, oolite, green-sand, lias, slate-clay of the coal formation, and carboni- 

 ferous limestone. 



N. lanceolata, Lam. Shell long transversely, thin, brittle, hyaline, 

 the sides before lanceolate, behind broad, obtuse Lam. vi. 1, 58. 

 This is the rarest of the genus. The valves have the form of the blade of a lan- 

 cet. 



N. rostrata, Lam. Shell transverse, oblong, slightly convex, thin,, 

 transversely striated ; anterior side much produced, curved, and 

 slightly ribbed longitudinally ; margin entire. ^ inch broad. 

 Coasts of Northern seas. B.~Mont. Test. Sup. pi. 27, fig. 7. 



N. margaritacea, Lam. (Area nucleus, Lin.) Shell somewhat tri- 

 angular, smooth, and the inside pearly ; apices recurved, approxi- 

 mated ; margin crenated ; epidermis olivaceous, glossy. ^ inch 

 long. European coasts. B. Brown's Illust. pi. 25, fig. 12. 



Gen. 11. PECTUNCULUS, Lam. Jrca, Lin. 

 Shell orbicular, equivalve, subequilateral ; hinge semicircular, 

 with numerous oblique teeth, those of the middle obsolete ; 

 ligament exterior. 



The shells of this genus are all marine, and approach the Pectenides in their 

 forms. Their internal margin is always crenated, and they have often longitudinal 

 radiated ribs. 



* With distinct longitudinal furrows. 



P. glycimeris, Lam. Shell suborbicular, transverse, subequilateral, 

 longitudinally sulcated and striated, gibbous, and thick ; umbones 



