380 MOLLUSCOIDEA phylum v 



In earlyforms thepedide aperture is usually covered hy deltidia and chilidia, but in most 

 later fonns these plates are lost. Fedicle always fundional 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. Ä prolific stock of Brachiopods. 

 Throughout Paleozoic. 



Family 1. Billingsellidae Scliucliert. 



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

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

 well developed. Ä spondylium is usually developed and to its upper surface are 

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

 dense, granulär, lamellar, rarely fibrous, apparently irregularly pundate in some 

 forms. Cambrian. 



Siibfamily A. Nisusiinae Walcott and Scliiichert. 



Primitive Billingsellidae 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 when present. 



Nisusia Walcott, and subgenus Jamesella Walcott. Distinctly plicate 

 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 Schucliert. 



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 well- 

 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 



Billingsella coloradoensis (Shumard). Middle Cambrian ; i pti • , i i • , • 



Texas. A, Ventral exterior. B, Ventral interior. C, Dorsal ^-UQ Uniua ; tnc gCnUS Qies OUt in 



interior. c, Crural bases ; d, Deltidium; j, Cardinal pro- fV,p T.n-«7-pr OrrlnviVinn TVip emh- 



cess ;«, Teeth ; vs, Vascular sinuses (after Walcott). tne l^OWer UrüOVlCian. lue SUD 



genus Otusia Walcott, has small 

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

 Upper Cambrian ; North America. 



Fin. 561. 



