126 PHOLADID”. 
closed behind; pedal processes short and curved; anterior 
margins reflected, covered by two small accessory valves; bur- 
row oval, lined with shell. 
Animal included within the valves, except the slender con- 
tractile siphons, which are furnished with pectinated ridges, and 
divided at the end ; foot thick, very extensile. 
Turnus, Gabb. 
Distr.—Cretaceous ; California, India. TT. plenus, Gabb. 
Shell like that of Xylophaga, but has posterior to the internal 
umbonal rib another, often broader rib, running from behind the 
apex to the infero-posterior margin. 
The animal also secretes a shelly tube, and consequently the 
valves must have been much more gaping posteriorly than they 
are in the recent Xylophage, which only slightly protrude out 
of their shells. The accessory valves are unknown, and it is 
therefore difficult to classify exactly the genus, for it may be 
just as possible that it is a form of the Teredinine. 
TuRNUS, Gabb. (Typical.) Shell transversely oval; anterior 
hiatus formed by the oblique truncation and slightly sinuous 
outline of the anterior ventral margins of the valves; umbonal 
sulcus and both of the internal ridges very oblique, narrow, 
smooth, and extending to the free margins. TZ. plenus, Gabb. 
GonlocHASMA, Meek. Shell transversely ovate-oblong ; hiatus 
formed by a deep rectangular notch in the anterior ventral mar- 
gins; umbonal sulcus and corresponding internal ridge slightly 
oblique, and the latter finely and obscurely crenate; posterior 
internal ridge broad, deep, very oblique, smooth; and not 
extending to the free margins. G. Stimpsoni, M. and H. 
XYLOPHAGELLA, Meek. Shell globose; anterior hiatus formed 
by a large, deep, rectangular notch in the anterior ventral mar- 
gins ; umbonal suleus and corresponding internal ridge descend- 
ing vertically, the latter being strongly crenate by little projecting 
points ; posterior internal ridge as in the last, excepting that it 
is less oblique and placed in front of the posterior umbonal 
slopes. X. elegantula, M. and H. (cv, 76). 
ZirpH#A, Leach, 1851. 
Distr.—3 sp. Europe, U. 8., Senegal, Straits of Sunda. Z, 
crispata, Linn. (civ, 61). 
Shell oval, cardinal margin scarcely reflected ; no accessory 
valves, the beaks protected by a membrane; usually a thin, 
fugacious epidermis; anteriorly greatly gaping. 
TaLona, Gray, 1847. 
Distr.—T. explanata, Spengler (civ, 62; ev, 84). W. Africa. 
Shell narrowly gaping anteriorly ; two accessory dorsal plates, 
