CONCHIFERA. 477 



Distrihution, 30 species. United States, France, Bed Sea, 

 India, New Zealand, Pacific, West America (Sitka — Peru), 

 Burrows in limestone and mud. 



Fossil, 20 species. Eocene — . United States, Europe. 



Glatjcomta (Bronn), Gray. 



Synonym, Glauconome, Gray, 1829 (not Goldfuss, 1826). 



Type, G. Sinensis, PI. XX., Fig. 18. [Glaucos, sea-green, 

 my a, mussel.) 



Shell oblong, thin ; epidermis dark, greenish ; ligament ex- 

 ternal ; hinge with 3 teeth in each valve, one of them bifid ; 

 pallial sinus very deep and angular. 



Animal yirith. a rather small, linguiform foot; pedal opening 

 moderate ; siphons very long, united, projecting far into the 

 branchial cavity when retracted, their ends separate and diverg- 

 ing; palpi large, sickle-shaped; gills long, rounded in fi-ont, 

 the outer shortest. 



Sub-genus, Tarysiphon, Benson. Differs from Glaucomya 

 in having the siphons united up to the end. 



Distrihution, 12 species. Embouchures of rivers; China, 

 Philippines, Borneo, India. 



Fossil, 2 species. Tertiaiy. Europe. 



Family XY. — MACTniDiE. 



Shell equivalve, trigonal, close, or slightly g-aping ; ligament 

 (cartilage) internal, sometimes external, contained in a deep 

 triangular pit ; epidermis thick ; hinge with 2 diverging car- 

 dinal teeth, and usually with anterior and posterior laterals ; 

 pallial sinus short, rounded. 



Animal with the mantle more or less open in front ; siphonal 

 tubes united, orifices fringed ; foot compressed ; gills not pro- 

 longed into the branchial siphon. 



Sections of the shell exhibit an indistinct cellular layer on the 

 external surface and a distinct layer of elongated shell. (Car- 

 penter.) 



Mactra, L. 



Etymology, mactra, a kneading trough. 



Synonyms, Trigonella, Da Costa (not L.), Schizodesma 

 (Spengleri), Spisula (solida), Mulinia (lateralis), Gray. 

 Type, M. stultorum, PI. XXL, Fig. 1. 

 Shell nearly equilateral ; anterior hinge-tooth A-shaped, with 



