DIELASMA 261 



it differs from the original illustration of that species, however, in the 

 absence of a mesial sinus in each valve. It more closely resembles 

 I), chouteauensis or D. fernglenensis than any of the other species here 

 described, but it is a proportionally narrower shell with more convex 

 valves 



Horizon. Lower Burlington limestone. 



DIELASMA BURLINGTONENSIS (White) 

 Plate XXXI, Figs. 45-49 



1860. Terebratula burlingtonensis White, Jour. Boston Soc. Nat. Hist., vol. 

 7, p. 228! 



Description. Shell below medium size, elongate-ovate in outline, the 

 greatest width near the mid-length, the anterior margin rather narrowly 

 rounded. The dimensions of a nearly perfect but slightly distorted speci- 

 men, a cotype, are: length of pedicle valve 16.' mm., length of brachial 

 valve 15.1 mm., greatest width 10.6 mm., thickness 7.5 mm. A large 

 pedicle valve with a width of 13.4 mm. has the anterior portion too in- 

 complete to allow the accurate measurement of the length which was 

 probably about 20 mm. 



Pedicle valve with its greatest depth posterior to the middle, arched 

 from beak to front with the convexity somewhat greater posteriorly, the 

 surface curving abruptly from the median line to the postero-lateral mar- 

 gins, becoming a little inflected to the cardinal extremities, curving more 

 gently to the antero-lateral and anterior margins; mesial sinus obsolete; 

 beak prominent, only slightly incurved, truncated by the very large, sub- 

 ovate foramen which encroaches wholly upon the umbonal region of the 

 valve, its margin fixing a plane which is either at a right angle to the plane 

 of the valve or even slopes anteriorly from the posterior to the anterior 

 margins of the foramen; the delthyrium large, its greater portion filled 

 by the beak of the opposite valve, its apex closed by a pair of deltidial 

 plates. Internally the foramen is bordered by a distinct pedicle collar 

 extending towards the interior of the valve ; the dental lamella are rather 

 short and widely separated. 



Brachial valve a little less convex than the pedicle, its greatest depth 

 posterior to the middle, the surface curving somewhat abruptly to the 

 postero-lateral margins and more gently to the antero-lateral and anterior 

 margins; mesial portion of the valve without fold or sinus; the beak 

 pointed and incurved beneath the margin of the deltidial plates of the 

 opposite valve. Internally the situation of the crural plates separate from 

 the socket plates, with the transverse, muscle-bearing plate joining the 

 inner surface of the valve medially, may be recognized from the position 

 of dark lines showing upon the external surface. 



Surface of both valves nearly smooth, marked only by very indistinct, 

 concentric lines of growth. 



