BRACHIOPODA 



181 



joined by a band near the umbo. Ordovician to Carboniferous ; 

 abundant in Silurian and Devonian. Ex. A. reticularis, Wenlock 

 Limestone. 



Fig. 83. Atrypa reticularis, Wenlock Limestone. A, dorsal valve show- 

 ing brachial skeleton. B, interior of ventral valve, a, impressions 

 of adductor muscles ; c, divaricator muscles; p, muscles of peduncle; 

 o, genital impression; d, deltidium. (From Woodward.) Natural 

 size. 



Fig. 84. Rhynchonella (Hemithyris) i^sittacea, Recent. A, interior of 

 ventral ; B, interior of dorsal valve. /, foramen ; d, deltidium ; t, 

 teeth; a, adductor impressions; r, divaricator impressions; p, pe- 

 duncular impressions ; o, genital impressions ; t', dental sockets ; 

 c, brachial skeleton; s, septum. (From Woodward.) Natural size. 



Rhynchonella (fig. 84). Shell triangular, or rounded, very 

 convex, not perforated by canals, usually ornamented with radiating 

 ribs. Usually a sinus on the ventral valve and a corresponding 

 ridge on the dorsal. Umbo small, acute, incurved ; foramen usually 

 enclosed by the deltidium. Ventral valve with two strong teeth, 

 muscular impressions in middle of valve. Brachial skeleton reduced 



