726 LAMELLIBRANCHIATA. 



ventricose, attenuated posteriorly, and having a concentrically fur- 

 rowed surface. There is a distinct "lunule," and the hinge is broad, 

 with strong cardinal teeth and feebly developed lateral teeth, while 

 there is a well-marked pit in the hinge-plate of each valve for the 

 reception of the internal ligament. The genus is marine ; the 

 earliest forms appearing in the Cretaceous rocks, while numerous 

 Tertiary and about thirty-five Recent species are known. The Cre- 

 taceous genus Crassatellina has been associated with Crassate/la, 

 but it agrees with the Astartidaz in having an external ligament. 



Order VI. Erycinacea. 



This order is defined by Fischer as comprising marine Bivalves, 

 with a byssiferous foot, and without respiratory siphons, but with the 

 exceptional character that the branchial aperture is placed anteriorly, 

 in front of the pedal opening, instead of being placed posteriorly 

 below the exhalant or anal aperture. The shell is small and thin, 

 equivalve, free, the hinge being toothed or toothless, and an internal 

 cartilage-pit and generally an external ligament being present. There 

 are two adductors, and the pallial line is entire. This order includes 

 the two small families of the Erycinidce and Galeommidaz. 



Family i. Erycinid^e. — The shell in this family is small and thin, 

 oval or trigonal in form, equivalve, mostly inequilateral, and smooth or 

 finely striated. The hinge carries one or two cardinal teeth, but lateral 

 teeth are not constant. The internal ligament is contained in an oblique 

 groove between the hinge-plates of the valves, and the external ligament 

 is feebly developed. 



The genus Erycina includes a large number of small shells which are 

 found in Tertiary deposits, and are specially abundant in the Eocene 

 rocks. A single species has been described from the Cretaceous rocks 

 of North America. Very closely related to Erycina, if generically separ- 

 able from it, is the genus Kellia, which is not known in the fossil condi- 

 tion (as distinct from the preceding). Related genera are Lepton and 

 Montacuta, both of which are known by Tertiary forms, and still survive. 



Family 2. Galeommidce. — In this family the shell is small, thin, equi- 

 valve, and subequilateral, and is more or less gaping. The hinge may 

 be edentulous, or may be provided with one or two teeth ; and the liga- 

 ment is internal, and is lodged in a median pit. All the forms included in 

 this family are marine, the two principal genera being Galeomma and 

 Scintilla, both of which are represented by living forms. The oldest 

 species of Galeoinma are found in the Pliocene deposits, while Scintilla 

 appears as early as the Eocene. 



Order VII. Cardiacea. 



The Lamellibranchs included in this order are inhabitants of the 

 sea or of brackish water, and possess a byssiferous or grooved foot ; 

 while there are two adductor muscles. (According to the views of 



