190 



hinge narrow, with numerous teeth like those of a comb ; 

 the beaks contiguous, inflected backwards ; the ligament 

 marginal, partly internal, inserted in a pit at the hinge. 

 Recent and fossil. PI. viii. fig. 18. 



Trigonice. With lamelliform teeth, transversely striated. 



1. Trigonia. An equivalved, inequilateral, trigonal, 

 and, sometimes, suborbicular bivalve ; the hinge teeth 

 oblong, flat on their sides, diverging, and grooved trans- 

 versely, the two on the right valve being grooved on both 

 sides, and received between the two on the left valve, 

 which are double and grooved only on the insides ; the 

 ligament external and marginal. Recent and fossil. 

 PL viii. fig. 3. 



2. Castalia. An equivalved, inequilateral, trigonal shell; 

 the beaks recurved ; the hinge with two teeth ; one back- 

 ward, short, and slightly trilamellated, with transverse 

 striae ; the other forward, longitudinal, and lateral ; ligament 

 external. Recent. 



A rare shell, apparently from fresh water. 



Naiades. 



Fresh water shells, having one irregular hinge tooth, 

 simple or divided, and one longitudinal tooth extended under 

 the corselet : sometimes, without any tooth ; but having, 

 along its base, irregular, granular tubercles. Impression 

 posteriorly compound ; furnished with an epidermis worn 

 off at the beaks. 



1. Unio. A transverse, equivalved, inequilateral, bi- 

 valve ; impression complicated ; the hinge with two teeth in 

 each valve ; one, short, irregular, simple or divided, and 

 substriated hinge tooth ; the other, lateral, compressed, and 

 elongated under the corselet ; ligament external. Recent 

 and fossil. PI. vii. fig. 19. 



