CONCHACEA. 



739 



shell is equivalve, slightly inequilateral, orbicular in form, and more or 

 less tumid. The hinge has usually two cardinal teeth in each valve, 

 without lateral teeth. The ligament is wholly or partially internal, and 

 the pallial line is entire. 



The type-genus of this little group is Ungulina, which is represented 

 by living species and by a few Tertiary forms. 



Family 6. Uxicardiid.e. — In this family the shell is equivalve, 

 slightly inequilateral, oval or rounded, and concentrically striated. The 

 hinge carries a single cardinal tooth in each valve ; the ligament is lodged 

 in an external marginal groove ; the adductor impressions are elliptical ; 

 and the pallial line is entire. The genus Unicardium is the type of this 

 family, and its species range from the Trias to the Chalk. Fischer 

 also includes here his genus Pscudedmofidia, of the Carboniferous rocks, 

 founded for the reception of certain forms of Edmondia in which the liga- 

 ment is external, and is placed in a marginal groove. 



Family 7. Taxcrediid.e. — This family includes transversely triangu- 

 lar shells, resembling Donax in form, with an external ligament, and an 

 entire pallial line. The hinge has in each valve two cardinal teeth (some- 

 times only a single tooth in one valve), with lateral teeth as well, at any 

 rate posteriorly. The type of this family is the extinct genus Tancredia, 

 in which the shell is attenuated in front, and obliquely truncated behind. 

 The species of this genus range from the Trias to the Chalk. 



Family 8. Doxacidje. — In this family the animal is marine or 

 estuarine in habit, with a large foot, and short separate siphons. 

 The shell is equivalve, more or less wedge- 

 shaped, close, and non -nacreous. The 

 hinge carries one or two cardinal teeth in 

 each valve, with inconstant lateral teeth ; 

 the ligament is external ; and the pallial 

 line is deeply indented. 



In the genus Donax (fig. 620), the shell 

 is wedge-shaped, the front rounded and 

 produced, and the posterior side short and 

 obliquely truncated. There are numerous living species of this 

 genus, and a small number of fossil forms, the oldest undoubted 

 types appearing in the Eocene deposits 

 Isodojita (Sowerbya) may be placed 

 in the neighbourhood of Donax. 



Family 9. Psammobiid.e. — This 

 family includes marine Bivalves, 

 with long separate siphons and a 

 tongue -shaped foot. The shell is 

 transversely elongated, equivalve, 

 sub-equilateral, slightly gaping at 



both ends, with an external ligament, and a deeply sinuated pallial 

 line (fig. 621). The hinge usually carries two cardinal teeth in 

 each valve, but there are no lateral teeth. 



The type of this family is the genus Psammobia, in which the 



Fig. 620. — Interior of the 

 right valve of Donax retusa. 

 Eocene. 



The Jurassic genus 



621. — Interior of the right valve of 

 Psammobia rudis. Eocene. 



