Report of the State Geologist, 



481 



bobthia the place of the delthvrium isoovered by a convexridgej 



in Acrotreta and C< there is a groove instead of a ridge. 



Fig. 66.— Pedicle-area of 

 Orbiculoidea E < 

 viewed from within. F, 

 foramen; p', under sur- 

 face of listrium. 



Fig. 57 —Interior of pedicle-valve 

 of Orbiculoidea nitida. F. 



foramen; y', inner surface of 

 pedicle-groove or listrium. 

 (Davidson.) 



iias 



been termed 

 c 



the 



Fio. 55.— Pedicle 

 of Orbiculoidea tenui- 

 lamellata F, fora- 

 men; (/, base of groove; 

 I, walls of groove. 



The convex ridge or plate which 

 deltidium in the articulate brachiopods 

 is retained in many genera at matu- 

 rity (Protorthis, Billing sell a, Stroph- 



E »DONTA, &TROPHOMENA, LePT,ENA,OrTHO- 



thetes, DERBVA,etc.,etc, Plates 8, 13, 19) 



and it usually exists in all species which 



have the cardinal area developed. 



That it is not always retained may be 



due to different causes. Its union with 



the margins of the delthyrium are lines 



of weakness along which it is of ten v™. ss.~ cutambomtes ccmmmho 



readily separated from the valves* it anomala - VA ' cardinal area of P edi- 



„___ -U j n t -, , ' cle valve; da, cardinal area of 



may be gradually abraded or become brachial valve; „, deitidium; c , 

 resorbed by the animal chiiidium; f, foramen. (DeVer- 



j „ ' NEUIL.) 



in some of the dentiferous genera in which the cardinal area 

 is very obscurely denned and the umbo of the pedi- 

 cle-valve closely incurved (Pentamer- s, Amphigenia), 

 the deltidium is concave, and the pedicle probably 

 not functional at maturity. 



In most of the articulate genera, however, 

 such as Terebkatula, Rhynchoxella and their 

 allies, Merista, Mebistella, Ketzia, etc, etc 

 the covering of the delthyrium consists of two ^_ 

 convex plates formed by gradual growth alone *»m ot cyrtina; 

 the margins of the opening and uniting in the cdI™ 

 median line; bounding the lower side of, or encircling 

 the pedicle. Various stages in the development of these 

 J-oyi. 61 33 



