CONCHIFERA. | 477 
Distribution, 30 species. United States, France, Red Sea, 
India, New Zealand, Pacific, West America (Sitka—Peru). 
Burrows in limestone and mud. 
Fossil, 20 species. Eocene—. United States, Hurope. 
GuAucoMYA (Bronn), Gray. 
Synonym, Glauconome, Gray, 1829 (not Goldfuss, 1826). 
Type, G. Sinensis, Pl. XX., Fig. 18. (Glaucos, sea-green, 
mya, 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 with 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 front, 
the outer shortest. 
Sub-genus. Tarysiphon, Benson. Differs from Glaucomya 
in having the siphons united up to the end. 
Distribution, 12 species. Embouchures of rivers; China, 
Philippines, Borneo, India. 
Fossil, 2 species. Tertiary. Europe. 
FAmMIty XV.—MACTRID. 
Shell equivalve, trigonal, close, or shghtly gaping; hgament 
(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. ) 
MactTrA, L. 
Etymology, mactra, a kneading trough. 
Synonyms, Trigonella, Da Costa (not L.), Schizodesma 
(Spengleri), Spisula (solida), Mulinia (lateralis), Gray. 
Type, M. stultorum, Pl. XXI., Fig. 1. 
Shell nearly equilateral; anterior hinge-tooth A-shaped, with 
