THE GENERA OF THE BRACHIOPODA. 
Il. 
BRACHIOPODA ARTICULATA. 
(CONTINUED.) 
Spirifer, Sowerby. 1815. 
(Plates 23-29.) 
Synonyms; Zrigonotreta, Koenig, 1825; Fusella and Brachy- 
thyris, McCoy, 1844. 
SHELLs transversely elongate, rarely produced axially ; with or 
without median fold and sinus. Hinge-line straight, usually form- 
ing the greatest diameter of the shell, but in some of the subdi- 
visions of the genus, short and inconspicuous. Cardinal extrem- 
ities alate, acuminate or rounded. 
Surface covered with granulations, striz, plications or costa, 
variously grouped on the lateral slopes and which may be pres- 
ent or absent on the median fold and sinus; these are crossed by 
concentric growth-linés which may take the form of varices or 
expanded lamella, or be modified into fimbriz of simple or com- 
pound spines. In the subgenera Martinis and Marrintopsis the 
surface is smooth except for the concentric striz. Shell substance 
fibrous, generally impunctate ; in the smooth species the epider- 
mal layer is minutely pitted. 
The pedicle-valve has the umbo more or less elevated over the 
hinge-line, the apex acute, erect or incurved. The cardinal slopes 
show a slight tendency to concavity or excavation, and the 
median portion of the valve is more or less strongly depressed by 
a sinus. The cardinal area is broad, flat or incurved and its 
surface is transversely striated; the inner shell-layers bear a 
series of longitudinal or vertical canals at whose marginal] 
extremities the fibrous tissue is sometimes produced into a row 
of denticles, corresponding to a row of pits on the opposite valve ; 
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