324 CAMBEIAN BRACHIOPODA. 



open cleft, or only so in the youngest-shelled stage, after which the ventral valve becomes 

 attached by a pedicle to foreign objects. Pedicle cleft in derived forms modified by a listrium. 

 Pseudodeltidia and pseudochilidia as a rule not well developed. 



Superiamily SIPHONOTEETACEA Walcott and Schucliert [Walcott, 1908e, p. 145]. 



Primitive, thick-shelled, calcareous or corneous, oboloid Neotremata, with the pedicle 

 passing through a ventral sheath, the aperture of which may remain apical and circular in 

 outline, or it may become elongate through resorption by passing anteriorly through the pro- 

 tegulum and umbo of the shell. A listrium is not developed. Dorsal protegulum marginal. 



Family OBOLELLID.a; Walcott and Schuehert [Walcott, 1908e, p. 145]. 



Primitive Siphonotretacea with the pedicle emerging through a small circular perforation 

 in the apex of the ventral valve, posterior to the protegulum. Derived out of the Obolidse. 

 Oholella Billings [1861b, p. 7]. 



(Glyptias) Walcott [1901, p. 675]. 

 Botsfordia Matthew [1891, p. 148]. 

 Schizopholis Waagen [1885, p. 752]. 

 (?) Quebecia Walcott [1905a, p. 320]. 



Family SIPHONOTRETIDa; Kutorga [1848, p. 2S3].a 

 (Emended Schuehert [1897, p. 130] and Walcott [1908e, p. 146].) 



Progressive Siphonotretacea with the circular or elongate pedicle opening at the apex or 

 passing by resorption anteriorly tlirough the protegulum and the umbo of the shell. 

 YorJcia Walcott [1897b, p. 714]. 

 Dearbornia Walcott [1908d, p. 78]. 

 Trematobolus Matthew [1893a, p. 276]. 

 ScUzambon Walcott [1884b, p. 69]. 

 Si-phonotreta de Verneuil [1845, p. 286]. 

 Keyserlingia Pander [1861, p. 46]. 



Superfamily ACBOTBETACEA Schuehert [1896, p. 308]. 



(Emended Schuehert [1897, p. 129] and Walcott [1908e, p. 146].) 



Progressive Neotremata with corneous or calcareocorneous shells. The pedicle opening 

 is a simple, circular, more or less conspicuous perforation through the apex of the ventral valve. 

 Dorsal protegulum marginal. 



Family ACE,OTRETID.a; Schuehert [1893, p. 150]. 



Same characters as superfamily. 



Subfamily ACROTHELINiE Walcott and Schuehert [Walcott, 1908e, p. 146]. 



Depressed large Acrotretidse. 



AcrotJiele Linnarsson [1876, p. 20]. 



(RedlichdU) Walcott [1908d, p. 89]. 

 Discinolepis Waagen [1885, p. 749]. 



Subfamily ACROTaETINiE Matthew [1903, pp. V4 and 91].!> 



Small Acrotretidse with more or less liigh ventral valves. 

 Linnarssonella Walcott [1902, p. 601]. 

 Acrotreta Kutorga [1848, p. 275]. 

 Acroihyra Matthew [1901b, p. 303]. 

 Discinopsis (Matthew MS.) Hall and Clarke [1892c, p. 105]. 



a Described as Siphonotretaese. ' Wrongly credited to Walcott and Schuehert [Walcott, 19D8e, p. 146]. 



