NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 1 5 



Diambonia sp. 1 

 Plate 2, figures 15, 16 



Description. — A single damaged pedicle valve has an outline more 

 transverse than that of other two species, being less than half as long 

 as wide ; specimen about 15 mm wide and 6.5 mm long ; cardinal angle 

 approximately 50° ; convexity of pedicle valve moderate ; surface 

 ornamented with 3 radial costellae which, at a distance of 3 mm from 

 the beak, are spaced about 2 mm apart. Pedicle interior with median 

 septum smaller than in other species. 



Discussion. — This specimen probably represents a species distinct 

 from the other two described here. The spacing of the radial costellae 

 is wider than in any previously described species. Only D. discuneata 

 (Lamont), among described species, has as wide an outline. The 

 convexity of the valve in lateral profile is remarkably low for the 

 genus. 



Figured specimen. — USNM 145341. 



Diambonia sp. 2 

 Plate 2, figures 18, 20 



Description. — Shell strongly concavo-convex ; cardinal angles acute, 

 forming an angle with the hinge line of a little over 60°. Brachial 

 valve bearing a very low, narrow fold, about 0.5 mm wide at anterior 

 margin. Pedicle valve possessing a correspondingly narrow sulcus, 

 in which a single costella is faintly developed; otherwise surface is 

 smooth. 



Measurements in mm: 





Hinge 



Length 



Length 



Cardinal 





width 



pedicle 



brachial 



angle 



USNM 145342 



6.2 



3.3 



2.8 



60° 



USNM 145342a 



5.6 



3.8 



— 



65° 



Discussion. — These are the only known specimens of Diambonia 

 with a surface almost completely devoid of ornamentation. Although 

 they may prove to be variants of one of the other species, they are 

 here conservatively separated until larger collections can be secured 

 and studied. One specimen of D. anatoli as described by Williams 

 (1962, pi. 16, fig. 24) resembles these Alaskan specimens to a re- 

 markable degree. 



Figured specimen. — USNM 145342. 



