BRACHIOPODA. 
87 
The muscular area consists of a long, ovate scar, which is divided into a 
subquadrate posterior pair, and a subcordate anterior pair of adductor impres¬ 
sions. These are separated longitudinally by a very faint median ridge. On 
casts of the interior the filling of the visceral foramen in the hinge-plate fre¬ 
quently shows a cross-striation like that of the pedicle-cavity of the opposite 
valve, and also indicates that the median ridge is continued throughout the 
extent of this passage. 
The surface of the valves is variously ornamented; in the typical group, at 
each concentric growth-line, there is a broad lamellar expansion; in some cases 
this expansion is striated longitudinally, or it may be divided into flat spines, 
which merge into the lamella at their bases; again the spines may be long and 
tubular, but connected by the laminar expansions. The surface frequently 
appears to be smooth, or covered only with concentric striae, and in one of the 
largest subdivisions of the genus (Seminula) this is a normal condition, while in 
other divisions it is often altogether casual. 
Shell-substance fibrous, impunctate. 
Type, Terebratula concentrica, von Buch. Middle Devonian. 
Observations. The number of species which, in common usage, are referred 
to Athyris, is very great. This name, like those of some other genera, 
Orthis, Strophomena, Atrypa, etc., has been a convenient receptacle for forms 
whose intimate relations were not thoroughly known; but the investigations of 
King, Davidson, Glass, Zugmayer, Bittner, and other careful students of the 
spiriferous brachiopods, have done much to eliminate from this association 
some of the more positively heterogeneous material. The diagnosis above 
given is restricted pretty closely to the essential characters of the well known 
species, Terebratula concentrica, von Buch, which, in the absence of any specified 
type, is usually, and quite properly regarded as the typical species, being 
it is the first in the list of descriptions accompanying the original account of 
the genus. McCoy applied the term Athyris to shells, which in his belief, 
possessed no .apical foramen or deltidium, but more careful observation soon 
showed that the concealment of the cardinal area was but a condition of 
