308 MOLLUSCOIDEA BRACHIOPODA SUB-KINGDOM v 



Thomasina, Hall and Clarke. Lingulae with the posterior margin of the 

 ventral valve notched, and with two conspicuous articulating processes. 

 Silurian ; France. 



Family 3. Lingulasmatidae. Winchell and Schuchert. 



Platform -bearing Lingulacea derived through Lingulidae. Ordovician and 

 Silurian. 



Lingulops, Hall. Small Lingulae with narrow, depressed, not excavated, 

 platforms. Ordovician and Silurian f North and South America. 



Lingulasma, Ulrich. Large thick-shelled Lingulae with very prominent, 

 slightly excavated platforms. Ordovician ; North America ; (?) England. 



Order 2. NBOTRBMATA. Beecher. 



Circular or oval, more or less cone-shaped, inarticulate Bmchiopoda, with the 

 pedicle opening restricted throughout life to the ventral valve. Pedicle aperture 

 modified by a deltidium or listrium. Prodeltidium attached to the ventral valve. 



Superfamily 1. ACROTRETACEA. Schuchert. 



Neotremata with phosphatic shells and a more or less well-developed deltidium . 

 Dorsal protegulum marginal. Cambrian to Silurian. 



Family 1. Acrotretidae. Schuchert. 



Acrotretacea with the pedicle opening posterior to the protegulum. Cambrian to 

 Silurian. 



Acrotreta, Kutorga. Ventral valve sub-conical ; the posterior margin 

 flattened, triangular in form, and resembling a true cardinal area. This 

 slope is divided medially by a shallow groove, widening downward. Circular 

 pedicle opening at the apex of the valve. Dorsal valve flat or depressed, 

 convex ; interior flanked by a large anterior and a smaller posterior pair of 

 muscular impressions ; exterior concentrically striated. Cambrian and Ordo- 

 vician ; North America and Europe. 



Conotreta, Walcott. Small, highly conical shells, with the interior of the 

 ventral valve marked by a number of radiating ridges. Ordovician and 

 Silurian ; North America. 



Acrothele, Linnarsson. Rounded, or broadly oval, depressed shells with 

 the cardinal area obsolete. Cambrian ; North America and Europe. 



Linnarssonia, Walcott. Smaller and more conical shells than Acrothele. 

 Cambrian ; North America and Europe. 



Discinopsis, Matthew ; New Brunswick. Iphidea, Billings ; North America 

 and Sweden. Volborthia, von Moller ; Russia. (?) Mesotreta, Kutorga ; Russia. 

 (?) Orbicella, d'Orbigny (Keyserlingia, Pander). (?) Helmersenia, Pander ; Russia. 

 All Cambrian. 



Family 2. Siphonotretidae. Kutorga. 



Acrotretacea with the pedicle opening passing by resorption anteriorly through the 

 protegulum and the umbo of the shell. Cambrian to Silurian. 



