372 



MOLLUSCOIDEA 



PHYLÜM V 



Helmersenia Paiider. Yery small Obolids with a pustulose or spiny 

 surface. Lower Ordovician ; Esthonia. 



Lingulella Salter (Eooholus Matthew) (Fig. 547). More or less elongate 

 Obolids. Widely distributed throughout the world. The following are 

 subgenera: Leptemholon Mickwitz, a.nd Lingulepis 

 Hall (decidedly elongate Obolids). '"^Jk P 



j 



„ . -, . . . ;_-_. . p"^ 



c 



Fig. 546. 



O&oZtts ma^MiaZis Hall. Middle Cambrian ; Minnesota. J, Ex- 

 terior of ventral valve. B, Interior of saine. C, Interior of 

 dorsal valve ; a, Filling of pedicle furrow ; c, Areaof outside lateral 

 scars ; Ä, Central scars ; i, Transmedian lateral scars ; j, Anterior 

 lateral scars ; ps, Parietal band ; v, Area of visceral cavity ; vs, 

 Vascular sinus with outside and inside vascular branches (after 

 Walcott). 



Fig. 547. 



Lingulella acutangulus (Roemer). 

 Upper Cambrian ; Texas. A, Interior of 

 ventral valve. />', Interior of dorsal 

 valve ; g, Umbonal scar ; h, Central scar ; 

 i, Transmedian lateral scar ; j, Anterior 

 lateral scars ; l, Outside lateral scars ; p, 

 Pedicle furrow ; ps, Parietal band ; pvs, 

 Vascular branches ; s, Median septum ; 

 X, Cordiform cavity; z, Vascular 

 branches (after Walcott). 



Delgadella Walcott. Has thickened internal margins. Lower Cambrian 

 of Portugal. 



Leptoholus Hall. Yery small Lingulellids of the Ordovician of North 

 America, in which the interiors are marked by two or three diverging, 

 slightly elevated septa, which occasionally are somewhat bifurcated terminally. 



Paterula Barrande (Cydus Barrande, non de Koninck 1841). Closely related 

 to Leptoholus, but the inner margins of the valves are thickened. Ordovician ; 

 Bohemia and North America. 



(?) Spondylobolus M'Coy. Generic characters not well known. Ordovician ; 

 Ireland. 



Subfamily B. Neobolinae Walcott and Schuchert. 



Progressive OboUdae with posterior platforms, to which were prohably attached the 

 central, outside and middle lateral muscles. Seems to have arisen in thick-shelled 

 Middle Camhrian Obolus, and is transitional to the platform-bearing Trimerellids. 

 Middle Cambrian. 



Neobolus Waagen {Lakhmina (Ehlert, and Davidsonella Waagen, non Mun.- 

 Chalmas 1880). Middle Cambrian of India. 





Fk; 



Elkania desiderata (Bill.). Upper 

 Cambrian; Quebec. J^, Interior of ven- 

 tral valve. B, Interior of dorsal valve ; 

 Ji, Central scars ; i, Transmedian lateral 

 scars ; j, Anterior lateral scars ; v, Pos- 

 terior lateral scars ; p, Platform with 

 muscle scars ; x, Vascular areas in front 

 of platform (after Walcott). 



Subfamily C. Elkaniinae Walcott and Schuchert. 



Divergent OboUdae with posterior or marginal 

 platforms, to which were attached the central, outside 

 and middle lateral muscles. Not in lineof develop- 

 ment to Trimerellids. Cambrian. 



Elkania Ford {Billingsia Ford 1886, non de 

 Koninck 1876) (Fig. 548). Cambrian; North 

 America. 



