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 Athtris, 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 Athtris 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 



