BRACHIOPODA. 305 



Diagnosis. Shells semicircular or transverse, usually normally concavo- 

 convex, sometimes plano-convex. Hinge-line straight, making the greatest 

 diameter of the shell. Pedicle-valve with a narrow concave or flat cardinal 

 area; the delthyrium more or less completely covered by a convex imperforate 

 deltidium. The upper margin of the area bears a single row of hollow vertical 

 or divergent spines, which increase in length toward the cardinal angles ; these 

 spines are the prolongations of tubes which penetrate obliquely the substance 

 of the shell in the cardinal region, converging toward the apex of the valve 

 till they reach the surface, where they turn at an abrupt angle upward and out- 

 ward and are thence continued as hollow spines. Cardinal teeth strong. A 

 low median ridge, slightly thickened at its posterior extremity, where it is 

 sometimes coalesced with the deltidium, extends forward, dividing the mus- 

 cular region. The muscular scars are usually faint, and consist of flabelliform 

 diductors which partially enclose elongate median adductors. 



In the brachial valve the cardinal area is very narrow, and without spines. 

 The deltidium is partially developed, resting against the cardinal apophysis 

 This process is very similar in character to that in Plectambonites, consisting 

 of a median portion, coalescing at its base with the elongate crural plates. On 

 its posterior face it is divided by a narrow median furrow and two broader 

 lateral grooves, giving it a quadrilobate appearance. The crural plates are 

 slightly divergent from the hinge-line, bounding narrow, elongate sockets. A 

 median ridge separates the quadruplicate muscular impression, and from between 

 the anterior and posterior members of this impression originate two linear 

 brachial ridges, which extend outward, recurving toward the median line at 

 about the middle of the valve, making a reniform curve. Interior surface of 

 both valves strongly papillose in the pallial region. 



External surface usually covered with radiating striae, rarely smooth or con- 

 centrically rugose. Shell-substance fibrous-punctate. 



Type, Orthis striatella, Dalman. Upper Silurian. 



Observations. Chonetes is remarkable for the persistence of its characters. 

 From its appearance in the middle Silurian to its disappearance in the Permian, 

 this type of structure has been maintained with few essential modifications. On 



