NATUEAL SCIENCES OF PIIILADELPHIA. 
27Ó 
1889.] 
tip uear thè siphonal opeiiing. Its niargiiis are smooth and siinple. 
Opposite tlie pedal opening a jiart of thè foot is produced and thick- 
ened, anteriori}' sharp edged, rounded behind, and having a spongv 
sole with a small deep (glandular ?) sinus near its anterior end ; this 
portion some observers would probably regard as constituting tlie 
whole foot, but to me thè view that it represents sometliing analogoiis 
to thè pedal disk of Pecten, seems more naturai. In thè sinus above 
thè jiosterior end of thè foot (as above defined) and in thè mediali 
line, is a slender subcylindrical process (about ó'O min. long as con- 
tracted in spirits) probably a sensory organ like that of Yoldia. 
Directly behind this is thè atrium of thè incurrent siphon, a small 
chamber hermetically roofed liy thè branchia which are uniteci by 
a thiu membrane so as to entirely cut off thè branchial from thè 
anal chamber. This membrane is continuous with, or firmly 
attached to, thè siphonal septum. Directly behind thè chamber 
thè passage-way or tube of thè sijihon contracts, though there seems 
to be no curtain valve. The branchia terminate at thè anterior end 
of thè siphonal septum. The gills fall in two doublé lamellie on 
each side. The inner gill is a simple elongated flattened sac, thè 
outer one has its inner wall dependent, then, at thè distai edge, 
folded back upon itself and continued upward, dorsally, beyond tlie 
point of starting, so that thè free edge forms a narrow “ appeudix. ” 
The viscera present no special points of interest. There is a large 
“hyaline stylet ” which is contained in a larger chamber extending 
nearly to thè front end of thè foot. The other end of thè stylet is 
attenuated, sharply bent, and projects iato thè cavity of thè 
stomach. The visceral massis large, thè intestine nuich convoluted, 
thè auus projecting, large, involute and cupshaped, constricted be- 
hind thè “ Clip.” The anterior adductor is normal but feeble ; thè 
muscle passes from thè reflected shell margin in one side straight 
across to thè other. The poster ior adductor is ovai in section and 
considerably stronger. In thè right valve is a slender, promineiit, 
compressed fossette carrying a small thiu, internai cartilage, which 
is attached to thè opposite valve upon a small sharp not-excavated 
shelly process. This remnant of thè originai cartilage may assist in 
keeping thè valves attached to one another, though it is very feeble, 
but it has wholly lost its originai function of an elastic cushion to 
prevent thè too sudden or rude closing of thè valves. It is doubt- 
ful if thè thiii shred of horny epidermis behind thè beaks in this 
