CLASS II 



BEACHIOPODA 



371 



delf hi/rial opening, rudimenfari/ articulation and immature homoeodeUidia. Muscle 

 scars prophetic of the Strophomenacea of the Protremata. Cambrian. 



A , B 



"1, 



Fig. '544. 



Schiichertina cuiuhria Walcott. Middle Cambrian ; 

 near Neihart, Montana. A, Interior of ventral valve. 

 B, Interior of dorsal valve (after Walcott). 



J 



Fig. r,i^,. 



KxUorgina r.intjiilata Bill. Lower Cambrian ; Vermont. 

 .!, Interior of ventral valve. /.', Side view of conjoined 

 valves. C, Interior of dorsal valve ; h, Central scars ; 

 j, Anterior lateral scar ; s, Median septum (after Walcott). 



Kutorgina Billings (Fig. 545). Lower Cambrian ; North America and 

 Sardinia. 



Superfamily 3. OBOLACEA Schuchert. 



Derived in Busfellacea. Progressive, thick-shelled, calcareo-jjhosphatic or corneous 

 Atremata 'without homoeodeUidia or pseudochilidia. Bounded or linguloid in outline, 

 more or less lens-shaped, and fixed by a short pedicle throughout life to extraneous 

 objects. Cambrian to Silurian. 



Family 1. Ourticiidae Walcott and Scliucliert, 



Primitive Obolacea with well-defined pedicle aperture common to both valves. 

 Interior characters much as in Obolidae. Middle Cambrian. 



Curticia Walcott. Middle Cambrian ; Wisconsin and Minnesota. 



Obolidae King. 



Family 2. 



Derived, progressive Obolacea with thickened, striated, verticcd cardinal areas 

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

 valves. Cambrian and Ordovician. 



Subfamily A. Obolinab Dall. 



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

 but never tending to form a sheath or to restrict the pedicle opening entirely to the 

 ventral valve. The most abundant Brachiopods of the Cambrian, vanishing 

 with the Ordovician. 



Obolus Eichwald ( Ungula Pander ; Ungulites Bronn ; Embolus Mickwitz) 

 (Fig. 546). More or less rounded Obolids. Widely distributed in Europe 

 and America. The following subgenera are recognised by Walcott : Broeggeria 

 Walcott, Upper Cambrian, England; Mickwitzella Walcott (Thysanofus Mickwitz, 

 non Alt. 1860), Ordovician, Esthonia ; Acritis Volborth {Aidonotreta Kutorga 

 1848, errore 1847), Ordovician, Esthonisi ; Schmidtia Volborth, Ordovician, 

 Esthonia ; Palaeobolus Matthew, Cambrian, eastern Canada ; Fordinia Walcott, 

 Middle Cambrian, Utah ; Lingulobolus Matthew {Sphaerobolus Matthew), 

 Upper Cambrian, eastern Canada ; Westonia Walcott (has transverse parallel 

 ornamentation). Middle Cambrian, Wyoming, Utah and Idaho. 



