NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 5 



(oral communication, 1963) as an immature Grewingkia. In the opin- 

 ion of Ross it indicates a late Middle or Late Ordovician age. 



TAXONOMIC DESCRIPTIONS 



Superfamily ORTHACEA Walcott and Schuchert, 1908 



Genus PTYCHOPLEURELLA Schuchert and Cooper, 1931 



Ptychopleurella cf. P. lapworthi (Davidson) 



Plate 1, figures 2, 4, 6, 8 



Orthis lapworthi Davidson. Davidson, 1883, Monograph British fossil Brachio- 

 poda, vol. 5, pt 2, Silurian Supplement, p. 176, pi. 13, figs. 9, 10, Paleonto- 

 graphical Society (London). 



Glyptorthis sublamellosa Cooper. Schuchert and Cooper, 1930, Am. Jour. Sci., 

 ser. 5, vol. 20, pp. 265-268, pi. 1, figs. 21-22. 



Description. — Shell biconvex, subrectangular, slightly wider than 

 long; greatest width just anterior of hinge line; anterior commissure 

 gently sulcate; costate with about 12-14 costae on each valve, strong 

 concentric growth lines provide lamellose appearance. Pedicle valve 

 evenly convex with low median fold bearing one strong costa, two 

 lateral costae are intercalated about one-third distance from pedicle 

 beak to anterior margin ; lateral slopes with 4-6 costae ; low apsaclinal 

 area has small open delthyrium ; dental plates reduced. Brachial valve 

 with rather shallow median sulcus bearing two costae; cardinalia 

 typical for genus with rather short, slightly curved brachiophores and 

 thin cardinal process. 



Discussion. — The Alaskan specimens agree closely with those illus- 

 trated by Davidson. They are also to a considerable degree like 

 Ptychopleurella sublamellosa (Cooper) from the Gaspe. The latter 

 species is slightly smaller and may have a deeper sulcus in the brachial 

 valve. However, size of costae and spacing of lamellae is highly 

 variable within our small sample and we consider P. sublamellosa a 

 possible synonym of P. lapworthi. Although it is difficult to establish 

 the range of variation within this group of shells because of the small 

 number of specimens — there are only eight specimens in the Alaskan 

 collection — there is enough variation to include not only the Gaspe 

 form but also P. uniplicata (Cooper) from the Porterfield. All these 

 specimens could represent a single species. More detailed studies of 

 variation, especially of the Virginia specimens, are needed before this 

 can be demonstrated. 



Figured specimens.— USNM 145324, 145325, 145326. 



