74 MISSISSIPPIAN BRACHIOPODA 



duced anteriorly as low ridges along the posterior half of the muscular 

 scar; no median septum; muscular scar ovate, extended anteriorly to be- 

 yond the mid-length of the valve ; the anterior margin crenulate internally. 

 Surface marked by numerous fine, but rather sharp, radiating costae which 

 increase by implantation, anteriorly the costae are subequal, but towards 

 the beak there is a series of stronger ones with from one to three finer 

 ones between. The costse are crossed by strong, irregular, concentric 

 lines of growth, varying in strength in different parts of the valve. 

 Braehial valve not known. 



Remarks. This species is known only from a single specimen, the holo- 

 type, which is a silicified pedicle valve wholly free from matrix, both ex- 

 ternally and internally, the valve is a little incomplete on the margin and 

 has lost the terminal portion of the beak. From a single example, in this 

 group of shells, the really essential specific characters can hardly be 

 pointed out, and the above description is therefore the description of an 

 individual rather than of the species. The characters most likely to be 

 constant and of specific value are the relative proportions of length and 

 breadth and the length of the hinge-line. Other examples, when found, 

 will doubtless vary in the details of outline and in the surface markings, 

 especially in the concentric markings of the shell. 



Horizon. Chester group. 



Genus ORTHOTETE8 Fischer de Waldheim 



Description. Shell subplano-convex to biconvex, either regular or more 

 or less distorted in manner of growth. The pedicle valve flat or convex, 

 the cardinal area rather high and sloping posteriorly from the hinge-line, 

 divided into a primary and secondary area as in Schuchertella and other 

 allied genera, the delthyrium closed to the apex by the deltidium. In- 

 ternally the dental lamellae are produced anteriorly towards the inner 

 floor of the valve and are joined to a median septum which sometimes 

 reaches nearly half the length of the valve. Towards the apex of the 

 beak is a small, triangular, pyramidal cavity or chamber formed by the 

 two dental lamellae internally and the inner surface of the deltidium ex- 

 ternally. The brachial valve convex, with a very narrow cardinal area 

 or with the cardinal area wanting. 



Remarks. The septate shells here included in the genus Orthotetes were 

 referred to Derbya first by Waagen 1 and later by Hall and Clarke, 2 the 

 name Orthothetes being used by these authors for a large group of non- 

 septate shells. It has been shown by Girty, 3 however, that the original 



iMem. Geol. Surv. India, ser. 13, vol. 1, p. 591 (1887). 



2 Pal. N. Y., vol. 8, pt. 1, p. 261 (1892). 



3 The Guadalupian Fauna, pp. 186-199 (1908). 



