14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



distance of 3 mm from the beak, are approximately 1 mm apart; 

 cardinal angle varies from 55° to 60° ; faint and narrow median sulcus 

 barely discernible and occupied by the median costella. Pedicle in- 

 terior with median septum shaped like a strong rod resting upon a 

 thin blade ; there is a suggestion that the supporting blade is subject 

 to resorption. No brachial valves obtained. 

 Measurements in mm: 



Hinge Cardinal 





Length 



width 



Thickness 



angle 



USNM 145340 



4 



7.5 



2.4 



±55° 



USNM 145340a 



4.2 



6.4 



2.4 



60° 



USNM 145340b 



4 



7 



— 



— 



USNM 145340c 



4 



7 



2.4 



+ 55° 



USNM 145340d 



3 



4 



1.6 



50° 



Discussion. — This species closely resembles D. anatoli Spjeldnaes 

 in size and proportions. Although ornamentation of the Scandinavian 

 species is not illustrated, Spjeldnaes (1957, pp. 80-81, pi. 2, figs. 6-8, 

 text fig. 11R) states that it possesses 5-7 radial costellae. It is not 

 clear whether the number is related to the size of the specimen. If 

 there is no relation it may be impossible to distinguish specimens of 

 D. anatoli with 5 costellae from the Alaskan specimens described here. 

 D. anatoli is found in Stage 4b alpha of the Norwegian sequence, 

 correlative with the Middle Ordovician Porterfield or Wilderness 

 Stages of North America (Berry, 1960, table 1) and with the lower 

 Caradoc of Great Britain (Dean, 1960, pp. 83-87). 



The Alaskan specimens compare closely in size with Diambonia 

 gibbosa (Winchell and Schuchert) (Cooper and Kindle, 1936, pi. 51, 

 figs. 9, 10). However, the Minnesota species is more nasute in an- 

 terior outline, has strong ridges defining the pedicle muscle area, and 

 possesses 6-7 radial costellae (Winchell and Schuchert, 1895, pp. 416- 

 417, pi. 32, figs. 13-17). 



Diambonia septata (Cooper) (Schuchert and Cooper, 1930, pi. 1, 

 figs. 9-13) is a relatively longer species, although it also has 5 simi- 

 larly spaced radial costellae. D. discuneata (Lamont) is a larger and 

 wider species than the Alaskan form, with main costellae spaced 

 0.5 mm apart. A form identified by Williams (1962, p. 173, pi. 16, figs. 

 25-28) as D. cf. D. discuneata from the upper Caradocian has cos- 

 tellae about 1.0 mm apart at a distance of 3 mm from the beak. Its 

 proportions seem closer to those of D. septata than to those of 

 Lamont's description of D. discuneata. 



Figured specimen. — USNM 145340. 



