BRACHIOPODA. 



117 



From their point of union a single process is extended backward at an 

 abrupt angle and with a very gentle downward curve, terminating just in front 



FIG. 104. FIG. 105. 



Two views of the.loop of fiimetrta Verneuiliana,ila.\\. (c.) 



of the apices of the primary lamellaB and above the bases of the spiral cones ; 

 its extremity is broadened and bifurcated, these secondary processes, however, 

 extending but a very short distance. The posterior edges of the lateral branches 

 of the loop and of the primary lamellae may be finely fimbriated. 



There is usually no trace of a median septum in this valve, but occasionally 

 an obscure ridge is preserved. No muscular markings have been observed. 



The external surface is covered with numerous fine radiating striae, which 

 are rarely crossed by concentric lines. Shell-substance abundantly punctate. 



Type, Retzia vera. Hall. Kaskaskia limestone. 



Observations. The term Eumetria was proposed as a designation for shells 

 which were believed to differ in the structure of the deltidium from forms at 

 that time regarded as typical of the genera Retzia and Rhynchospira. In treat- 

 ins: of the genus Rhynchospira it has been shown that the coalescence of the 

 deltidial plates may occur in the early forms of that genus, as it does also in 

 Retzia and Nucleospira, but nowhere does it become so well marked and not- 

 able a character as in the group of fossils under discussion. It was also shown 

 in the original description of the genus Eumetria, that the umbonal cavity of 



