48 AXEL A. OLSSON 



Dcba. Shell generally small, rounded to corbuliform, rostrated, the pallial 

 line simple. Chondrophore small. 



33. Shell small, rounded or depressed, with noded radial riblets. In- 

 terior bright and pearly. 



Verticordidae 



34. Shell corbuliform, elongated, the anterior side short, rounded and 

 convex, the posterior side narrowed, produced, and pointed. 



Cuspidariidae 

 Dcbb. Shell small or medium-sized, the chondrophore larger, the pallial 

 sinus sinuous. A lithodesma often present. 

 Dcbba. Mostly nestlers, the shell often quite irregular in shape. 



35. Shell sometimes paper thin or heavy, generally with a peeling 

 periostracal cover. 



Lysonidae 

 Dcbbb. Not nestlers, the shell with a more regular form, the periostracum 

 more closely adherent. 



36. Shell subporcellaneous and often quite thin, inequivalve, the right 

 valve larger, surface with rude sculpture, earthy, or pustulose. 



Thraciidae 



37. Valves hatchet-shaped, generally depressed, with a pearly surface. 

 Hinge with strong crural ridges. 



Pandoridae 



38. Shell usually rounded, the right valve larger and convex, the left 

 smaller and depressed, pearly. Hinge without crural ridges, the 

 chondrophore a large, spoon-shaped process. 



Periplomatidae 

 Dd. Hinge margin without any teeth, and the ligament is atrophied or 

 obsolete. Dorsal margin smooth, reflected, or rolled over the um- 

 bone to which it may be closely appressed. Borers in rock, wood, 

 shell, or burrowing deeply in a sand or clay bottom. (Adesmacea). 

 The shell is generally white, thin, open or gaping at the ends and 

 along the dorsal and ventral margins or these spaces may be cov- 

 ered by accessory calcareous plates. There is generally within the 

 umbonal cavity a slender projecting arm or myophore (apophysis) 

 to which the foot muscles were attached. 



39. Typically borers in wood, the shell small and much reduced in 

 size. Bore lined with a heavy calcareous, wormlike tube, generally 

 much contorted. 



Teredinidae 



40. Shell larger and more perfectly formed, with open gaps or these 

 spaces covered with accessory plates. Shell thin, the surface 

 marked with coarse, scabrous ribs usually set apart in well-defined 

 areas. Borers in rock, wood, shell, or simply in the sea bottom. 



Pholadidae 



