312 MOLLUSCOIDEA BRACHIOPODA SUB-KINGDOM v 



Superfamily 1. STROPHOMENACEA V Schuchert. 



Primitive Protremata without spondylia and cruralia. Cambrian to Recent. 



Family 1. Kutorginidae. Schuchert, 



Primitive Strophomenacea with incipient cardinal areas, great delthyrium and 

 very rudimentary articulating processes and deltidium. Cambrian. 



Kutorgina, Billings. K. cingulata, as redefined by Walcott, from the 

 American Lower Cambrian, is the only species positively known to belong to 

 this family. Schizopholis, Waagen, of the Lower Cambrian in India, and 

 F'olborthia, Moller, of the Ordovician, may also possibly find a place here. 



1 Family 2. Eichwaldiidae. Schuchert. 



Primitive or aberrant, rostrate Strophomenacea, with narrow lateral grooves and 

 ridges for articulation. Delthyrium closed by a concave plate (? deltidium). Pedicle 

 emerging through the ventral umbone and moving with growth anteriorly by resorption 

 through the shell, as in Siphonotretidae. Ordovician and Silurian. 



Eichwaldia, Billings. The single species of this genus has a smooth 

 exterior. Ordovician ; North America. 



Dictyonella, Hall (Eichwaldia, auct.) Exterior surface of valves pitted in 

 quincunx, resembling Trematis. Silurian ; North America, England, Bohemia, 

 and Gottland. 



Family 3. Billingsellidae. Schuchert. 



Strophomenacea with well-developed cardinal areas and deltidium. Cardinal 

 process obsolete or very rudimentary. Articulation fairly well developed. Cam- 

 brian. \ 



Billingsella, Hall and Clarke (Protorthis, H. and C.) This is the only known 

 genus of the family, and contains a number of the oldest well-developed 

 species of Protremata. Cambrian ; North America. 



Family 4. Strophomenidae. King. 1 



Strophomenacea with well-developed cardinal areas, deltidium, chilidium, cardinal, 

 and articulating processes. Ordovician to Permian. 



Piafinesquina, Hall and Clarke (Fig. 517). Shells normally concavo-convex 

 dorso-ventrally. Striae alternating in size, and crossed by finer concentric 

 growth lines. Muscular area of ventral valve consisting of two broad flabel- 

 late diductor scars enclosing an elongate adductor. In the dorsal valve, the 

 bilobed cardinal process is low ; the posterior arborescent adductor scars 

 well defined. Vascular and ovarian markings often well indicated. Ordovician; 

 North America and Europe. 



1 [According to J. M. Clarke, a separation into groups of the genera here included under one 

 family is advisable, according as the shell is normally convex, or reversed convex. The habit of 

 reversion, originating in the unequal growth of the two valves, characterises a large number of forms, 

 which stand in a notable degree in successive genetic relationships ; while their affinities witli the 

 normally convex forms are less palpable. This author is in favour of restricting the Stropkomenufaf 

 so as to include only those genera in which the shell is usually reversed convex, and employing the 

 family term Leptaenidae for those having normally convex shells. TRANS.] 



