68 
the gili chambers due to the suspension of the giil axes which are 
free, except for their ends. By means of the anal slit the gili 
chambers communicate with the exterior. In their finer structure 
the giils, as already mentioned, are smooth, not folded. There are, 
however, many interlamellary connectives, though limited to the 
marginal parts of each demibranch. In the margin of each reflected 
lamella a marginal venous vessel is contained. 
The front margin of each demibranch is furnished with a fine 
longitudinal furrow. 
The foot consists of an anterior fingershaped portion and a 
broader posterior one, the latter containing the large byssus cavity 
preceded by a mucus giand which debouches in the anterior end 
of the byssus groove, 
Both pairs of labial palps are well developed. There is a long 
oesophagus. The stomach chiefly consists of two portions: a large 
cardiac and a smaller pyloric sac, the latter evidently homologous 
with the crystalline coecum, though containing only ciliated epithel- 
ium and no excretion. The former portion is lined with a stomachal 
cuticula and is dorsally furnished with a short curved coecal ap- 
pendage. Only one liver duet is present on each side of the 
stomach under the entrance of the oesophagus. The intestine issues 
from the right basal part of the pyloric sac, describes a short coil 
towards front and then runs with a dorsal curve through the heart 
to the anus. 
The gonads form a pair of few-lobed bunches in the posterior 
half of the visceral hump. They debouch in a pair of papillae on 
each side of the foot retractors within the inner giil room and 
beneath the fundus of the pericardium. 
Behind the pericard and surrounding the foot retractors lie the 
nephridia which begin with long pericardial tubes. These debouch 
dorsally in the posterior part of the outer sacs. The latter com¬ 
municate medially by means of a short opening in their walls. They 
debouch separate from and opposite to the gonads. 
Kidderia costata n. sp. 
(PI. II, figs. 45—47.) 
Shell small, ovate-oblong, thin, fragile, compressed, especially 
below the middle, very inequilateral; umbones broad, flattened, in 
