FAMILY OSTRACISE ANOMIA 167 



T. caput-serpentis, Linn, Gould. (T. septentrionalis, Couthouy, Bost. Journ. Vol. 2, p. 65. Pl. 

 34, fig. 321 of this work.) Shell rather thin, semitransparent, ovate: upper valve truncated hori- 

 zontally at the apex ; foramen large, one side completed by the apex of the lower valve ; surface 

 with a downy epidermis, under which minute radiating stnoe. From under each tooth in the lower 

 valve arises a thin process, curving a little inwards, whose extremities support an oval partially 

 twisted ring: margin of the shell crenate. Color, whitish. Length, 0*4; Width, 0-2. Coast 

 of Northern Europe, Maine and Massachusetts. 



T. psittacea, Gmel. (Gould, Op. cit. p. 142, fig. 91. Pl. 34, fig. 322 of this work.) Shell thin and 

 fragile, subtriangular, narrowed above ; the beak produced into a decurved horn : surface striated 

 concentrically and in radii ; foramen triangular. Color, brownish black or sea-green. Length, 

 0*35; width, 0*25. Northern Europe, and Seacoast of Massacusetts. 



T. thalassina. Gould. 



SECTION 2. LAMELLIBRANCHIA. 



Animal adherent, enveloped in a bilobed mantle, varying in the number and dimensions of 

 its apertures. Mouth transverse, medial, concealed at the bottom of the mantle between 

 two pair of appendices. Gills in the form of semicircular leaves, composed of two pair, 

 one on each side of the body : vent posterior and medial. Shell composed of two valves 

 connected by a hinge and ligament, and enclosing the animal. 



FAMILY OSTRACIDjE. 



Animal with the mantle not adherent, entirely open except on the dorsal part, without tube 

 or peculiar opening. Foot wanting or rudimentary . The two pair of gills united in a 

 medial line. Shell inequivalve, inequilateral, irregular, more or less lamellar or foliated : 

 hinge variable ; ligament internal or partly internal ; muscular impression single, sub- 

 central. 



GENUS ANOMTA. Bruguieres. 



Animal with the edges of its mantle thin, and furnished with a series of tentacular filaments. 

 Foot rudimentary ; the adductor muscle divided into three branches, the largest of which 

 passes through an aperture in the lower valve, with a corneous opercle to attach itself to 

 other bodies. Shell thin, often translucent : one valve convex ; the other flattened or con- 

 cave, and perforated near the beak. Ligament of the hinge short and thick ; muscular 

 impression tripartite. 



