126 PHOLADiD-a:. 



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. T. plenus, Grabb. 



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 posteriorlj'^ than they 

 are in the recent Xylophagse, 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 maybe 

 just as possible that it is a form of the Teredininse. 



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. T. plenus, Gabb. 



GONiocHASMA, 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. Cf. Stimpsoni^ M. and H. 



XYLOPHAGELLA, Meek. Shell globose ; anterior hiatus formed 

 by a large, deep, rectangular notch in the anterior ventral mar- 

 gins ; umbonal sulcus 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, 16). 



ZiRPH^A, Leach, 1851. 



Distr. — 3 sp. Europe, U, S., 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, 1841. 

 Distr, — T. explanata^ Spengler (civ, 62 ; cv, 84). W. Africa. 

 Shell narrowly gaping anteriorly; two accessory dorsal plates. 



