46 INTRODUCTION. 



gillum, Clavagella, Teredina, Teredo, Xylophaga, Fis- 

 tulana, and Gastrochaena. 



2. Pholadaria (plates, fig. 55 to 59), cylindrical, living in 



holes in rocks pierced by the animals. Lamarck places 

 in this family the genera Pholas and Gastrochaena, the 

 last of which belongs more properly to the family Tubi- 

 colse, as placed above. 



3. Solenacea (plates, fig. 60 to 68), longitudinally (trans- 



versely, Lam.) elongated, open at the anterior and pos- 

 terior extremities. This family contains the genera 

 Solen, Pholadomya, Panopaea, Glycimeris (Solecurtus) 

 and Solenimya. 



4. Myaria (plates, fig. 69 to 76), ligament internal. A 



spoon-shaped ligamentary pit in one or both valves. 

 Shell generally gaping at one or both extremities. This 

 family includes the genera Anatina, Mya, Anatinella, 

 Lyonsia, Myochama, Cleidotherus. 



5. Mactracea (plates, fig. 77 to 88), the cartilage placed in 



a trigonal pit, with a small external ligament. The 

 genera Lutraria, Mactra, Crassatella, Erycina, Ungu- 

 lina, Amphidesma, and Solenimya belong to this family, 

 the last of which ought to have been placed among the 

 Solenacea, as above. 



6. -Corbulacea (plates, fig. 89, 90), inequivalve, with an in- 



ternal ligament resembling the Mactracea, but differing 

 in having one valve deeper than the other, although 

 regular shells. This small family contains only the 

 genera Corbula and Pandora. 



7. Lithophagidce (plates, fig. 91 to 97), irregular, terebrating, 



living in holes of rocks. The genera are Saxicava, 

 Petricola, and Venerirupis. 



8. Nymphacea (plates, fig. 98 to 110), ligament external, 



generally placed upon a prominent fulcrum, which 



