HAMBURGIA 283 



plates are rapidly reduced in height and soon become obsolete, beyond 

 which point the hinge-plate is not connected with the inner surface of 

 the valve, but becomes a concave plate joining the bases of the crura 

 and terminating anteriorly in a short distance. The complete form of the 

 brachidium is not known, but it is probably short, not reaching the 

 mid-length of the valve. 



Remarks. This genus is perhaps most closely allied to Cranwna, from 

 which it differs in the extreme concavity of the hinge-plate, the cavity 

 between it and the inner surface of the valve being much reduced in 

 height; accompanying the great depression of the hinge-plate is the 

 absence of its perforation at the apex, which is, perhaps, the most diagnos- 

 tic character. The genus is totally distinct from Dielasma, in which the 

 crural plates originate as ridges upon the inner surface of the valve 

 instead of upon the concave surface of the hinge-plate ; the concave trans- 

 verse plate between the bases of the crura is somewhat similar in the 

 two genera except that it is not connected along its median line to the 

 inner surface of the valve in Hamburgia, but in Dielasma the inner surface 

 of this plate furnishes attachment for the adductor muscles which ap- 

 parently is not true in Hamburgia. 



HAMBURGIA TYPA "Weller 

 Plate XXXI, Figs. 16-18 



1906. Cryptc-.nella ? sp. undet. Weller, Trans. St. Louis Acad. Sci., vol. 



16, p. 443. 

 1911. Hamburgia typa Weller, Jour. Geol., vol. 19, p. 446, figs. 6a-h. 



Description. Shell below medium size, subovate in outline, longer than 

 wide, the greatest width near the mid-length, the postero-lateral margins 

 nearly straight or gently convex, the antero-lateral and anterior margins 

 semicircular. The dimensions of two detached pedicle valves are : length 

 17.8 mm. and 15.8 mm., width 14.8 mm. and 12 mm., convexity 4.7 mm. 

 and 4.4 mm. The dimensions of a brachial valve are: length 14 mm., 

 width 11.9 mm., convexity 4.4 mm. 



Pedicle valve with the greatest convexity near or a little posterior to 

 the middle, the surface arched from beak to front along the median line 

 either nearly symmetrically or with a slightly increasing convexity pos- 

 teriorly, the slope gently convex from the median line to the lateral mar- 

 gins, becoming a little inflected towards the cardinal extremities ; mesial 

 sinus obsolete, but the mesial portion of the valve is sometimes slightly 

 flattened along an indefinite band ; beak truncated, not strongly incurved, 

 with a large, subcircular foramen which encroaches wholly upon the um- 

 bonal region, only the apex of the delthyrium being in contact with it ; 

 the delthyrium broadly triangular, completely closed by a pseudodel- 



