146 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Family SIPHONOTRETID^ Kutorga, 1848 (emend) 



Progressive Siphonotretacea with the circular or elongate pedicle 

 opening at the apex or passing by resorption anteriorly through the 

 protegulum and the umbo of the shell. 



Yorkia 



Dcarbornia 



Trcniatobolus 



ScJ\izaiuhon 



Siphonotrcta 



Kcyserliiigia 



Superfamily ACROTRETACEA Schuchert, 1896 (emend) 



Progressive Neotremata with corneous or calcareo-corneous shells. 

 The pedicle opening is a simple, circular, more or less conspicuous 

 perforation through the apex of the ventral valve. Dorsal protegu- 

 lum marginal. 



Family ACROTRETIDzE Schuchert, 1893 

 Same characters as superfamily. 



Subfamily ACROTHELIN^E Walcott and Schuchert, new 



Depressed, large Acrotretidse. 

 Acrothele 



(Redlichella) 

 Discinolcpsis 



Subfamily ACROTRETIN.E Walcott and Schuchert, new 



Small Acrotretida; with more or less high ventral valves. 

 Liimarssoiiclla 

 Acrotreta 

 AcrotJiyra 

 Disciiiopsis 



Superfamily DISCINACEA Waagen, 1885 



Derived Neotremata with phosphatic shells, a listrium modifying 

 the pedicle slit, and without pseudodeltidia and false cardinal areas. 

 Dorsal protegulum usually subcentral. 



Family DISCINID^E Gray, 1840 



Discinacea with an open pedicle notch in early life in the pos- 

 terior margin of the ventral valve, which is closed posteriorly during 



