260 PROCEEDING'S OF THE NATIONAL MUSEUM. vol. xxvin. 



extending to the luteral niar<riiis; .-lualler ribs are intercalated l^etwecn these. On 

 each wide of tiic jinmii of ril)s on tlie niiddle third of the i^hell are four fainter, hnt 

 more strongly an: hed groups of ridges, wiiicii throw off minor ridges at the l)ack, 

 directed toward tiie lateral margins. On the ventral valve the small ribs are almost 

 all inter<'alated and ilo not spring from the back of the i^rimary ribs, as in those of 

 the dorsal valve. 



Size: Length, 12 mm.; width, 14 mm.; height of area of dorsal valve, 2 mm., of 

 ventral valve, 1 mm. A valve sujiposed to be of this species is 15 mm. long and 19 

 mm. wide. 



Horizon and lucaUty. — In dark shales of division ;^a, at Navy Island. 



Obser options. — With the available data this species appears to be 

 quite as near to Plectorthis as to an}' described genus. It does not 

 appear to belong with tlie typical forms of Stropliomena or Rafines- 

 quhui. The surface ribs, increasing by interpolation, and possibly 

 l>ifurcation, and form of the pseudospondylium in the ventral valve, 

 strongly suggest relationship with PUctorthis. It will probably 

 require better preserved material to satisfactorily determine the cor- 

 rect generic reference for the species. 



Foriiiation and locality. — Upper Cambrian. Division 3a of Matthew 

 section, St. John terrane, Navy Island, St. John Har))or, New Bruns- 

 wick. 



PLECTORTHIS CHRISTIANIiE Kjerulf. 



Ortltis cJiristianiir K.ieri'lf, Veiviser ved. geol. Exk, i Christiania, 1865, pp. 1,.3, 



tigs. 8a, b, c. 

 Ortlus cJiristiaiiiir Brogger, Die Sil. Etagen 2 nnd H, 1882, p. 4S, pi. x, figs. 14a, 



b, c. 

 OrtJm christianiir Gagel, Physik. Oekon, Gesellschaft-Konigsberg. Brach. Camb. 



und Sil. Diluvium Ost uud Westpreussen, 1890, p. 84, pi. 11, figs. 22a, 22b. 

 Ortlm chrislkmix Pompeckj, Neues Jahrbuch, Min. Geol. und Pal., I, 1902, 



Tremadoc der Mont. Noire, p. 7. 



This shell appears to differ from an}^ described species by the pecul- 

 iarity of the bifurcation of the radiating ribs. In all the species I 

 have referred to Plectortlds the increase in the number of ribs is by 

 interpolation and not by true bifurcation. In O. christianim the 

 increase is by both methods. The reference to Plectorthh is some- 

 what doubtful, as there is not sufficient data to base a reference on the 

 characters of the interior, cardinal areas, or convexity of the valves. 



I have received a large number of specimens from Norway and 

 Sweden lal^eled (Jrthis cJiristianice. After a study of all available 

 material only the shells from the argillaceous shale of Russelokken, 

 Norwa3\ and Oiand, Borgholm, Sweden, appear to belong to the 

 species. The shells in the Ceratopyge limestone {O. daunua) have 

 ribs that incirease in number by interpolation of ribs in the interspaces 

 between the older ribs, and a second species {P. wiinani) that occurs 

 in both the shale and limestone has simple ribs with few interpolated 

 beyond the umbo. Another transverse form has simple strong ribs 

 that increase in numlxM- by interpolation of a few new ribs. 



