16 GENERA OF SHELLS. 



II. LIGAMENT APPEARING OUTSIDE ; OR, BEING DOUBLE, 



HAS ONE INTERNAL, AND THE OTHER EXTERNAL. 



UNGULINA. Shell longitudinal, or subtrans- 

 verse, rounded above, subequilateral ; valves 

 closed, beaks decorticated, a short, sub-bifid 

 cardinal tooth in each valve, with an oblong 

 marginal pit adjacent, contracted in the mid- 

 dle ; ligament interior, inserted in the pits. 



The ungulinee are furrowed externally, and tinged 

 red on the inside. 



U. oblonga, transversa. 



SOLENOMYA. Shell inequilateral, equivalve, 

 transversely oblong, obtuse at. extremities ; 

 margined by a projecting, shining epidermis, 

 beak not prominent, scarcely distinct; a dila- 

 ted, compressed, very oblique cardinal tooth 

 in each valve, slightly concave above, receiv- 

 ing the ligament ; ligament partly interior and 

 partly exterior. 



The solenomyae resemble the mya at first sight, but 

 are allied by their characters to the solen, and still 

 more to the anatina. They are thin shells, almost 

 cylindrical, with diverging rays from the beaks to the 

 border and extremities of the valves. 



S. australis, mediterranea, velum. 



AMPHIDESMA. Shell transverse, inequilate- 

 ral, suboval or rounded, sometimes a little gap- 

 ing at sides. Hinge with one or two Jeeth, and 

 a narrow fossa for the interior ligament ; liga- 

 ment double ; one exterior, short, and one in- 



