464 MISSISSIPPIAN BRACHIOPODA 



beneath the covering of the delthyrium of the opposite valve. Internally 

 a strong median septum extends anteriorly from the beak for about one- 

 third the length of the valve. 



Surface of both valves marked by moderately strong, somewhat irregu- 

 larly distributed, concentric lines of growth. 



Remarks. In the original definition it is stated that only two examples 

 of this shell had been found by the author of the species. These two 

 original examples, with two additional ones in the collection of Walker 

 Museum, are all that have come under the observation of the writer. Of 

 Rowley's two co-types, one, the figured one, has the shell entirely pre- 

 served, but the second example is partially exfoliated and distinctly shows 

 the median septum of the brachial valve. Of the two Walker Museum 

 specimens, the larger and more complete one, whose dimensions are given 

 above, and which has served mainly as the basis of the above description, 

 is partially exfoliated so that the median septum of the pedicle valve is 

 clearly exhibited, and the smaller is an internal cast throughout, so that 

 the mesial septa of both valves are clearly seen. Rowley's figured type 

 has been carefully examined, and although the shell itself is intact, the 

 presence of a median septum in each valve seems to be clearly indicated. 

 The specimen here illustrated is somewhat broader, proportionally, than 

 the figured type. 



Horizon. Lower Burlington limestone (white chert). 



Family ATHYRIDJE 



Genus ATHYRI8 McCoy 



Description. Shells usually of medium size or larger, transversely sub- 

 elliptical, subcircular or subovate in outline, the valves subequally con- 

 vex, the median sinus of the pedicle valve and the fold of brachial valve 

 usually limited to the anterior portion of the shell and sometimes obso- 

 lete. In the pedicle valve the beak is usually incurved so as to conceal 

 the foramen and delthyrium, deltidial plates usually absent. Internally 

 the hinge-teeth are prominent, recurved at the tips, and are supported by 

 short but rather stout dental lamella? which are not produced anteriorly 

 about the muscular area; between the dental lamella? is a deep, trans- 

 versely striated pedicle cavity, and in front of this an ovate muscular area 

 extending about one-half the length of the valve ; the diductor scars are 

 flabellate and usually ill-defined about the margins, and the adductors 

 occupy a narrow central area more sharply defined. In the brachial valve 

 the dental sockets are broad and deep, the socket walls are connected by 

 a hinge-plate which is perforated posteriorly, close to the beak of the 

 valve, by a circular opening, the median portion of the plate is flat or con- 

 cave, the lateral margins are thickened and are produced anteriorly into 



