BIVALVES. ARCA. 59 



the margin entire; others, again, are crenulated 5 and se- 

 veral have prominent angular slopes, which give the shell 

 an auriculated appearance ; the anterior slope is, however, 

 by far the most prominent of the two. 



This genus presents but little beauty, though some of its 

 species are considered rare. 



The Ark is found in the European, Indian, American, 

 and Atlantic oceans: the Baltic, Northern, and Red seas 

 also produce some species. 



ARCA Ark. 



DIVISION I. WITH THE TEETH OF THE HINOE IN A STRAIGHT 

 LINE. 



FAMILY 1 Shell twisted. 

 Tortuosa Twitted. 



FAMILY 2. Shell rhomboidal. 

 Nose Noah's. Candida White. 



*Imbricata Imbricated. Indica Indian. 



Navicularis Boat. *Lactea Milky. 



Plicata Plaited. Reticulata Reticulattd. 



FAMILY 3. Shell oblong or transversely ovate. 

 Magellanica Magellanic. Bicolorata Party-coloured. 



Lacerata Hairy. Modiolus Muscle. 



Fusca Brown. Corbula Basket. 



Senegalensis Senegal. 



FAMILY 4. Shell subcordate. 



Lavigata Smooth. Granosa Grained. 



Pella Brittle. Rhombea Rhomboid. 



Antiquata Antiquated. Senilis Rugose. 



FAMILY 5. Gaping. 



Complanata Compressed. Nivea White. 



Barbata Bearded. Cancellata Cancellated. 



