380 MOLLUSCOIDEA phylum v 



In early forms the pedicle aperture is usually covered by deltidia and chilidia, but in most 

 later forms these plates are lost. Pedicle always functional and in the great majority 

 of forms emerges freely out of the delthyrium. Cardinal process more or less well 

 developed except in the most primitive genera. A prolific stock of Brachiopods. 

 Throughout Paleozoic. 



^a" 



Family 1. Billingsellidae Schuchert. 



Primitive Orthacea with a more or less closed, or an open delthyrium. A 

 cardinal process arises in this family and is therefore either absent, rudimentary or 

 well developed. A spondylium is usxially developed ami to its uj)per surface are 

 attached the muscles of the ventral valve. Cruralia rudimentary. Shell structure 

 dense, gramdar, lamellar, rarely fibrous, apparently irregularly punctate in some 

 forms. Cambrian. 



Subfamily A. Nisusiinae Walcott and Scliucliert. 



Primitive Billing sellidae with more or less well -developed deltidia and with or 

 without rudimentary chilidia. Spondylia and cruralia rudimentary or small, not 

 supported by septa. Cardinal process generally absent, but rudimentary ivhen present. 



Nisusia Walcott, and subgenus Jamesella Walcott. Distinctly jilicate 

 Billingsellidae without cardinal process. Deltidium well developed with an 

 apical pedicle foramen. The genus has a spiniferous exterior while the sub- 

 genus is devoid of spines. Lower and Middle Cambrian of America and 

 Europe. 



Protorthis Hall and Clarke. Has a spondylium, but the deltidium is widely 

 open for the protrusion of the pedicle. Shell substance apparently punctate. 

 Middle and Upper Cambrian of America and Sweden. The subgenus Loperia 

 Walcott, differs in having the ventral umbo high and convex while the rest 

 of the shell is concave. Middle Cambrian ; Cape Breton, Nova Scotia. 



Subfamily B. Billingsellinae Walcott and Schuchert. 



Primitive Billingsellidae very much like the Nisusiinae, but without true spondylia 

 (i.e. pseudospondylia are often present) and cruralia. There is a more or less tvell- 

 developed simple cardinal process except in Lower Cambrian forms. 



Billingsella Hall and Clarke (Fig. 561). Shells essentially orthoid, plicate, 



biconvex or planoconvex, and prob- 

 ably punctate. Deltidia well de- 

 veloped, but chilidia only partially 

 so; the pedicle may emerge be- 

 tween these plates or pass apically 

 through the deltidium. Common 

 and widely distributed throughout 

 the Cambrian of America, Europe 



BUlingsdhi coloradoeiuls (Sh\\xn9.vA). Middle Cambrian ; -i pii • .i j- , ■ 



Texas. A, Ventral exterior. B, Ventral interior. C, Dorsal ^^'^ Unilia ; tnc genUS QICS OUt 111 



interior. cCnual bases; f^ Deltidium; j^ Carclinal pro- ^he LoWCr Ordoviciau. The Sub- 



cess ; t, Teeth ; ■i'.«, Vascular sinuses (after Walcott). 



genus Otusia Walcott, has small 

 eared forms without deltidia and with a well-developed cardinal process. 

 Upjier Cambrian ; North America. 



Fiii. 561. 



