300 MISSISSIPPIAN BRACHIOPODA 



rather small, pointed and incurved; cardinal area of moderate height, 

 concave with the curvature increasing towards the beak, the lower and 

 flatter portion lying at nearly a right angle to the plane of the valve, the 

 lateral margins well defined but rounding rather abruptly to the lateral 

 slopes of the valve ; delthyrium about as wide as high ; each lateral slope 

 marked by five or six rounded or subangular, simple plications, which 

 originate along the cardinal margin, those bounding the mesial sinus are 

 the strongest, the others becoming successively smaller to the cardinal ex- 

 tremities. Internally a strong median septum reaches from the beak to 

 more than one-third the length of the valve, the hinge-teeth are supported 

 by dental plates which diverge anteriorly and extend about one-third as 

 far as the median septum. 



Brachial valve nearly or quite as convex as the pedicle, the greatest 

 depth near the middle or towards the front of the mesial fold ; the surface 

 convex on each side of the fold, usually becoming a little compressed 

 towards the cardinal extremities; mesial fold rounded or subangular, 

 sharply defined, moderately or rather highly elevated in front; the beak 

 small and incurved ; the cardinal area very narrow, lying in nearly the 

 plane of the valve ; the plications on the lateral slopes are similar to those 

 of the opposite valve and alternate with them. 



Surface of both valves, when well preserved, marked by crowded tuber- 

 cles which are the bases of spinules which are scattered in position or 

 sometimes are arranged in rather irregular radiating rows. One or more 

 rather strong lines of growth are sometimes present towards the anterior 

 margin of the valves. Shell substance punctate. 



Remarks. This species occurs commonly in some of the Chester faunas 

 and can be easily recognized by the character of the surface markings. 

 It may be easily distinguished from 8. transversa, with which it is some- 

 times associated, by its proportionally narrower and longer shell, by the 

 absence of a median plication in the fold and sinus, and by its surface 

 markings. 



Horizon Chester group 



Genus DELTHYRIS Dalmaii 



Description. Shell small or of medium size, spiriferoid in external form, 

 the mesial fold and sinus non-plicate or with a single median plication, 

 the lateral slopes of the valves with simple, rather strong plications, both 

 valves marked by concentric, lamellose lines of growth of greater or less 

 strength. Pedicle valve with an arched cardinal area of moderate height, 

 and an open delthyrium ; internally the dental lamellae are well-develpoed 

 and between them is a well-defined median septum. The brachial valve is 

 similar to Spirifer internally, with a brachidium as in that genus. 



Remarks. The genus Delthyris commonly has not been recognized above 

 the Devonian, but there is a group of small Spiriferina-\ike shells in our 



