A SYNOPSIS OF 



4. MYA.— Plates. 



Animal, an Ascidia. 



Shell, bivalve, generally gaping at one end. Hinge, for the most part, 

 with a single solid, thick, broad tooth, strongly attached by carti- 

 lage, but not inserted into the opposite valve. 



5. SOLEN.— Plate 3. 



Animal, an Ascidia. 



Sliell, bivalve, equivalve, elongated transversely ; open at both ends. 

 Hinge with an awl-shaped reflex tooth, often double ; not in- 

 serted into the opposite valve. 



6. TELLINA.— Plate 3. 



Animal, a Tethys. 



Shell, bivalve, generally flattish, sometimes orbicular, anterior end 

 often produced into an angular beak. Hinge usually with three 

 teeth ; lateral teeth in one of the valves flat ; often remote. 



7. CARDIUM.— Plate 5. 

 Animal, a Tethys. 



Shell, bivalve, equivalve, nearly equilateral, somewhat heart-shaped, 

 ribbed longitudinally ; beaks prominent, valves toothed or plaited 

 on their internal margin. Hinge with four teeth in each valve ; 

 the two cardinal teeth near together, and alternating with the op- 

 posite ; the two lateral teeth remote and inserted. 



8. MACTRA.— Plate 6. 



Animal, a Tethys. 



Shell, bivalve, valves unequal at the sides ; subtriangidar ; beaks pro- 

 minent. Hinge with the middle tooth spoon-shaped, having a 

 small cavity on each side ; lateral teeth remote, locking into the 

 opposite valves. 



