478 MISSISSIPPIAN BRACHIOPODA 



portion of the pedicle valve. This coloration in the fossil shell is perhaps 

 due to some original coloration in the living shell. 

 Horizon. Burlington limestone. 



CLIOTHYRIDINA TENUILINEATA (Rowley) 

 Plate LXXX, Figs. 1-12 



1900. Athyris tenuilineata Rowley, Am. Geol., vol. 25, p. 264, pi. 5, figs. 



31-33. 



1901. Cleiothyris hirsuta Weller, Trans. St. Louis Aead. Sci., vol. 11, p. 



187, pi. 16, figs. 25-27. 



Description. Shell small, lenticular, subcircular in outline, the length 

 and breadth subequal, the greatest width near the mid-length of the shell, 

 the hinge-line much shorter than the greatest width, the cardinal ex- 

 tremities rounded. The dimensions of two nearly perfect examples, 

 metatypes, are: length of pedicle valve 8.9 mm. and 8 mm., length of 

 brachial valve 8.1 mm. and 7.5 mm., greatest width 8.7 mm. and 8.7 

 mm., thickness 5.1 mm. and 4.8 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the cardinal margin on each side of the beak, and more gently 

 to the lateral and anterior margins; mesial portion of the valve slightly 

 flattened anteriorly but not depressed to form a mesial sinus ; the beak 

 rather small, sharply pointed and incurved, perforated by a subcircular 

 foramen which encroaches upon the umbonal region of the valve ; the 

 delthyrium filled by the beak of the opposite valve. 



Braehial valve subequally or a little less convex than the pedicle, the 

 greatest convexity posterior to the middle, the surface curving abruptly 

 to the cardinal margin and more gently to the lateral and anterior mar- 

 gins, the umbonal region rather prominent and projecting posteriorly a 

 little beyond the hinge-line; mesial fold absent but the median portion 

 of the valve is slightly flattened towards the front; the beak strongly 

 incurved beneath that of the opposite valve and nearly or quite filling 

 the delthyrium of the pedicle valve. 



Surface of both valves marked by thin, closely crowded, regular, 

 imbricating lamellae which are divided into flattened spines, of which 

 three or four occupy one millimeter, the spines of successive concentric 

 rows are usually uniform in position and are consequently rather regularly 

 arranged in radial series; when partially exfoliated the shell is marked 

 only by the fine concentric markings. 



Remarks. This species is so closely allied to C. hirsuta of the higher beds 

 of the Mississippian series that there is perhaps no justification in con- 

 sidering them as distinct. A careful examination of all examples avail- 

 able, however, seems to indicate that the concentric rows of flattened 

 spines in this species are somewhat finer, more regular and more closely 



