BRACHIOPODA. 



119 



ported by dental plates in the ventral valve. Brachial 

 skeleton often filling a great part of the interior of the 



FIG. 38. Spirifer striatus, Carboniferous. A, interior of dorsal valve ; 

 showing brachial skeleton. B, interior of ventral valve. (From 

 Woodward.) 



shell, in the form of two spiral ribbons, with their apices 

 directed laterally. Silurian to Permian. The sub-genus 

 Spiriferina occurs in the Lias. 



Uncites. Shell elongate-oval, biconvex, striated. 

 Hinge-line curved, no area. Umbone of the ventral valve 

 prominent and incurved, foramen closed in the adult. 

 Apex of dorsal valve recurved and partly hidden in the 

 ventral valve. Brachial skeleton spiral, the apices of the 

 spires directed laterally. Devonian. 



Atrypa (fig. 39). Shell sub-circular, biconvex, orna- 



A B 



FIG. 39. 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, ovarian impression ; d, deltidium. (From Woodward.) 



