igiS- ^O. 12. THE STROPHOMEXipAJE'Or THE KRISTIANIA REGION. 43 



Palaeostropbomena new genus. 



General form as in Strophomtna and Strophonelia but differs distinctly 

 in interior characters of dorsal valve, the cardinal process being not bilobed 

 but as in the Orthidae simple, having the shape of a short vertical plate, 

 posteriorly united with the crural plates, which end rather abruptly. The 

 cardinal process is supported by a faint median elevation in the valve 

 stret-ching for some distance anteriorly. Interior of ventral valve not 

 known. Type species | = only ^)ecies yet known) Palaeostrophomaaa amcaxfa 

 F. Schmidt. 



Palaeostropbomena concara F. Schmidt. 

 (PI. YU, fig. I— 21- 



1858. S/j ofAouMtm tU ÊM mu m F. S cm uui, Untere über die silnosctie Formation Tt>n Ehst- 

 land, Nord-Lirland and Oesel. Ardnr for die Nalnrknnde Liv^-. Eh^- and Kur- 

 lands, I. series, voL 11, p. 217. 



188a. Stïïo pam mi aÊm rhom&OÊdmUs Wnxx. var., Broggek. Die ScL Et. 2 und 3. p. 50, pL XI, 



fig- 5- 



M a : . pres. Two fairly complete dorsal valves in rock besides 

 minor fragments. 



Description. Shell of medium size, outline rcunded ;r .-^- îr 

 with greatest width at hinge line. Cardinal angles moderately acute, about 

 70®. The dimensions of tiie best preserved specimens were: width about 

 25 mm., length 19 mm., about the same si2e being attained by another 

 specimen, while a third one is somewhat larger. Circumbonai part in 

 dorsal valves neariy flat, in one ^>ecimen distinctly concave near the beak. 

 At a distance from the hinge of about -/s of the total length of die shell, 

 a distinct geniculation appears, with an angle as seen in the specimen 

 best preserved of about 130^. 



The radiating striae are extremely fine, and crowded- In a space of 

 I mm. about 5 occur. The striae are all of fairly equal size except for 

 one or two or three tiiat are much more prominent than the res:. 



These ribs are broadly rounded in section and increase in strengüi 

 towards the front Furthermore in the central disc we have strong, irre- 

 gular, concentric corrugations. Indications of concentric wrinkles may 

 also be seen in the deflected marginal portion. 



I have removed the shell substance with acid in the two most com- 

 plete of roy specimens and thereby found the interior in these dorsal 

 valves to be entirely different from that found in ordinary Slropkomena- 

 species of younger beds, as far as concerns the cardinal process. In the 

 median- plane we find in the mould an elongate narrow cavit\- (see pi- MI, 

 fig. 2 1, no doubt corresponding to the former process. 



