322 CAMBRIAN BRACHIOPODA. 



Order ATREMATA Beeclier [1891, p. 354]. 



(Emended Schuchert [1897, p. 119] and Walcott [1908e, p. 142].) 



Primitive inarticulate corneous or calcareophosphatic Brachiopoda with the pedicle 



emerging more or less freely between the two valves. Growth takes place in general around 



the anterior and lateral margins. SpeciaUzed forms show tendency to develop rudimentary 



articulation. Delthyrium originally unmodified, in later genera modified by pseudodeltidia 



and pseudochUidia, or by thickened, striated, and more or less furrowed or even cleft vertical 



cardinal margins, the ventral cleft in most specialized forms tending to inclose the pedicle and 



finally restrict it to the ventral valve; when completely so the genera are referred to the order 



Neotremata. 



SuperiamUy RTISTELLACEA Walcott [1908e, p. 143]. 



Primitive, thick-shelled, corneous or calcareophosphatic Atremata developing more or 

 less of pseudodeltidia and pseudochilidia. 



Family RUSTELI,ID.ff; Walcott [1908e, p. 1431. 



Primitive Rustellacea with the delthyrium small, open, and not much modified by pseudo- 

 deltidia or pseudochilidia. Muscle scars and vascular sinuses not well defined in the shell. 

 Rustella Walcott [1905a, p. 311]. 



Family PATERIinD.a; Schnchert [1893, p. 151]. 

 (Emended Schuchert [1897, p. 119] and Walcott [1908e, p. 143].) 

 Progressive Rustellacea with the delthyrium more or less closed by pseudodeltidia or 

 pseudochilidia. 



Mickwitzia Schmidt [1888, p. 24]. 

 Micromitra Meek [1873, p. 479]. 



iPaterina) Beecher [1891, p. 345]. 

 . (IpUdella) Walcott [1905a, p. 305]. 

 VolhortUa Kutorga [1848, p. 277]. 

 {VjEelmersenia Pander [1861, p. 48]. 



SuperiamUy OBOLACEA Schuchert [1896, p. 305]. 



(Emended Schuchert [1897, p. 119] and Walcott [1908e, p. 143].) 

 Derived (in Rustellacea), progressive, thick-shelled, calcareophosphatic or corneous Atre- 

 mata without pseudodeltidia and pseudochihdia. Rounded or Itnguloid in outhne, more or 

 less lens-shaped, and fLxed by a short pedicle throughout life to extraneous objects. 



Family CTJRTICIID^ Walcott and Schuchert fWalcott, 1908e, p. 143). 



Primitive Obolacea with a high well-defined delthyrium. Interior characters much as in 

 Obolidse. 



Curticia Walcott [1905a, p. 319]. 



Family OBOLID.a: King [1846]. 

 (Emended Schuchert [1897, p. 119] and Walcott [1908e, p. 143].) 

 Derived, progressive Obolacea with thickened, striated, vertical cardinal areas traversed 

 by pedicle grooves. Muscles and vascular trunks strongly impressed in the valves. 



Subfamily OBOLIN^ Dall [1870, pp. 154 and 161). 



(Emended Walcott [1908e, p. 143].) 



Primitive Obolidse with the pedicle grooves more or less shallow or deeply rounded, but 

 never tending to form a sheath or to completely restrict the pedicle opening to the ventral 

 valve. The radicle of the Trimerellidse, by way of the Neobolin^, appears to be in this sub- 

 family in the thick-shelled Middle Cambrian forms of Obolus (s. s.). 



