CLAss II BRACHIOPODA 37 1 



delthyrial opening, rudimentary articulation and immature homoeodeltidia. Muscle 

 scars prophetic of the Strophomenacea of the Protremata. Cambrian. 



A B 



l'Ki. '.J44. 



Fig. 545. 



^, , , ,. , . w , .. ,r-,j, ^ Kutonßna cAngulatuBm. Lo wer Cambrian ; Vermont. 



bckuchertina amhna Walcott. Middle Cambrian ; A, Interior of ventral valve. B, Siele view of conjoined 



near Neihart, Montana. ^, Interior of ventral valve. valves. C, Interior of dorsal valve; h, Central scars- 



L, Interior of dorsal valve (after Walcott). 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 Eustellacea. Progressive, thick-shelled, calcareo-phosphatic or corneous 

 Atremata without homoeodeltidia or pseudochilidia. . Bounded or linguloid in outline, 

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

 objeds. Cambrian to Silurian. 



Family 1. Ourticiidae Walcott and Scbuchert. 



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

 Interior characters mucli as in Obolidae. Middle Cambrian. 



Curticia Walcott. Middle Cambrian ; Wisconsin and Minnesota. 



Family 2. Obolidae King. 



Derived, progressive Obolacea with thicJcened, striated, vertical cardinal areas 

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

 valves. Cambrian and Ordovician. 



Subfamily A. Obolinae; Dali. 



Primitive Obolidae with the pedicle grooves more or less shallow oi' 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 Eidhwald ( Ungula Pander ; Ungulites Bronn ; Euobolus 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 (Thysanotus Mickwitz, 

 non Alt. 1860), Ordovician, Esthonia; Acritis Volborth (Aulonotreta Kutorga 

 1848, errdre 1847), Ordovician, Esthonia; 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 transversa parallel 

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



