Maryland Geological Survey 419 



each side of the medial sinus, and of several similar ones upon the corre- 

 sponding mesial elevation. Since these specimens indicate the first de- 

 parture toward the Aperturati spirifers so common in the Devonian and 

 Carboniferous formations, it seems desirable to give them a distinct name. 

 Therefore for the S. concinnus forms with the fold and sinus plicated 

 the name S. proavihis is proposed. The type is the S. concinnus Hall and 

 Clarke. 



Occurrence. — Helderberg Formation, Becraft Member. Warren 

 Point, Pennsylvania. 



Collection. — Maryland Geological Survey. 



Spirifer concinnoideus n. sp. 

 Plate LXXII, Fig. 4 



Description. — This subquadrate Spirifer has its nearest relationship 

 in S. concinnus of the Helderberg (Becraft). It attains twice the size of 

 the latter, is somewhat less convex, the dorsal medial fold less high, the 

 ventral sinus less deep, and it is flattened along the center, while the de- 

 pressed subangular plications range in number from fourteen to seventeen 

 on each side of the center against twelve to fourteen in S. concinnus. The 

 surface detail is not preserved in the material at hand, but being a sub- 

 quadrate shell the plications were probably covered by a series of fine 

 radial pustulose striiE devoid of imbricating lamellge. The general form is 

 very much like that of 8. concinnus, with a vertical incurved ventral car- 

 dinal area of moderate size. 



The subquadrate form of the shell readily separates S. concinnoideus 

 from all the alate forms like S. intermedius, while the great number of 

 depressed and >;mall plications separates it from S. perdewi and S. gordoni. 



Length 2.8 cm. ; width 4 cm. 



Occurrence. — Oriskany Formation, Ridgely Member. 21st Bridge, 

 Maryland; north fork of South Branch of Potomac River, Pendleton 

 County, West Virginia. 



Collection. — U. S. JSTational Museum. 



