NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS — ROSS AND DUTRO 1 3 



of prominent vascular trunks originate close to the brachiophores ; 

 one pair runs close on either side of the median septum, crosses the 

 circumvisceral ridge, and branches dichotomously twice ; the other 

 main pair nearly bisects the visceral area and crosses the boundary 

 ridge before branching. Interior surfaces of both valves covered with 

 fine papillae, probably representing inner ends of pseudopunctae. 



Discussion. — Additional collections from the area of Hard Luck 

 Creek will undoubtedly produce more specimens of this species. Until 

 we have a larger sample on which to base a new species it seems best 

 to refer these forms tentatively to Anoptambonites grayae (David- 

 son). 



Figured specimens.— USNM 145334, 145335, 145336. 



Genus DIAMBONIA Cooper and Kindle, 1936 



This genus is probably represented in the Alaskan collection by 

 three species. No brachial interiors have been found so that these 

 specimens cannot be distinguished from Bilobia. However, associa- 

 tions in the Perce fauna suggest that Diambonia is more likely to be 

 present and all three species are assigned tentatively until brachial 

 valves are found. 



The three species differ markedly in shell ornamentation. Two 

 specimens possess a single radial costella {Diambonia sp. 2) ; 5 speci- 

 mens have 5 radial costellae spaced about 1 mm apart (D. cf. D. 

 anatoli), and one specimen has 3 radial costellae spaced over 2 mm 

 apart (D. sp. 1). Although all three may be variants of a single 

 species, the total sample studied is too small to permit a reasonable 

 appraisal of variability until more extensive collecting has been done. 

 Among these specimens a tendency for the smaller ones to possess a 

 larger median septum than the large specimens suggests that septa 

 in the larger specimens may have been resorbed. 



Although Diambonia is typically a Middle Ordovician genus, D. 

 discuneata (Lamont) is reported from Lower Ashgill rocks (Lamont, 

 1935, p. 316), and D. septata (Cooper) is reported from Perce from 

 strata reputedly of Late Ordovician age. 



Diambonia cf . D. anatoli Spjeldnaes 

 Plate 2, figures 10, 12 



Diambonia anatoli Spjeldnaes. Spjeldnaes, 1957, vol. 37, No. 1, p. 80, pi. 2, 

 figs. 6-8, text fig. 11R. 



Description. — Pedicle valve strongly convex, wider than long with 

 acute cardinal extremities ; ornamented with 5 costellae which, at a 



