INTRODUCTION. 47 



passes from the inside to the outside of the hinge; 

 valves generally gaping at the extremities. This family 

 contains the genera Sanguinolaria, Psammobia, Psam- 

 motoea, Tellinides, Corbis, Lucina, Donax, Capsa, and 

 Crassina. 

 9. Conchacea (plates, fig. Ill to 121), regular, having 

 several cardinal teeth and sometimes lateral teeth. The 

 Conchacea constitute one of the most beautiful and 

 numerous families of the class ; they present equivalve 

 shells, which are always regular, unattached, and in 

 general closed, especially at the sides ; they are always 

 more or less inequilateral. They are divided into the 

 fiuviatile and marine Conchacea, the first containing the 

 genera Cyclas, Cyrena, and Galathaea, found in rivers ; 

 and the second, Cyprina, Cytherea, Venus, and Vene- 

 ricardia. 



10. Cardiacea (plates, fig. 122 to 130). This family, which 



resembles the last in some general characters, are also 

 regular and equivalve, and are generally provided with 

 radiating ribs, which are seldom seen in the Conchacea. 

 The genera enumerated in this family are Cardium, 

 Cardita, Cypricardia, Hiatella, and Isocardia. 



11. Arcacea (plates, fig. 131 to 138). These are known by 



having a row of numerous small teeth on the cardinal 

 hinge in each valve. The genera included are, Cucullsea, 

 Area, Pectunculus, Nucula. 



12. Trigonacea (plates, fig. 139 and 140). It is doubtful 



whether this family should remain distinct. As of the 

 two genera placed in it, the first, Trigonia, is thought by 

 some naturalists to have strong affinities with Nucula, 

 in the family of Arcacea ; and the latter, Castalia, cer- 

 tainly belongs to the Nayades. 



13. Nayades (plates, fig. 141 to 152). These are fresh water 



