I915 No. 12. THE STROPHOMEMDAE OF THE KRISTIANIA REGION. 99 



further development of the latter ? As far as can be judged from the 

 rather poor Norwegian material supplemented by the information found 

 in Davidson's illustrations of similar forms, the answer is in the negative. 

 The first mentioned form in my opinon has near relations in Rafinesquinas, 

 among others in the common R. expansa. The convexity- of the younger 

 form is greater no doubt, and the striation somewhat different, but these 

 are no very important differences. The interior characters of the ventral 

 valves are ver}' much alike, and the interior of the dorsal ones, as they 

 are depicted by Davidson from specimens of 5"/;'o/>/;o«/r;/a g.v/)a;wrt i Davidson 

 pi. XLV, fig. 10) and Sirophouioia imbrex. (?) var. semiglobosa (pi. XLI, 

 fig. 3) are exceedingly similar. Davidson's figures of S. expausa show- 

 also, which has not been distinctly observed in the Norwegian material, 

 that that form has smooth hinge margins, in other words is a Rafiuesquina. 

 As has been mentioned previously R. expausa must be considered closely 

 related to R. allernata, the t^'pe species of Rafiuesquina. 



As to the Brachypnon IValnistedti. this is a form that in general 

 shape and surface markings shows a good deal of likeness to a form from 

 an older horizon, viz. Rafiuesquina (?) semipartila Roem. from 5, but the 

 interior characters of the two forms are not, however, well enough known 

 to permit an}- certain conclusion as to their real relationship. 



Of resupinate Strophomenids with crenated hinge margins, we have 

 two, where the denticulation was directly observ-ed, Amphistrophia Davidsoni 

 and A. cnglyphoides. 



The former, as was mentioned previously (p. 92), so far from being of 

 a new type, in external appearence as well as in interior characters of 

 the dorsal valve (those of the ventral one not known) comes very close 

 to an older form with smooth cardinal areas, the StropJiontena Brøggeri 

 from 4 b. Amphistrophia euglyphoides does not come especially near to any 

 of the Strophomenas found in older beds, but on the other hand does not 

 show any peculiar characters besides the crenulated cardinal margin, that 

 distinguish the form from the older resupinate Strophomenids as a 

 whole. The species has much likeness to the American Clinton form 

 Strophomena {Strophonella ?) patenta Hall, the ventral cardinal area of which 

 according to existing illustrations is smooth. 



Strophofiella euglyplia that is known from other regions to have well 

 defined denticulations on the inner hinge margins also possesses no exterior 

 on interior features. — the latter can be seen from Davidson's illustrations — 

 that differ considerably from those of older types. The crural plates are 

 less well developed than in the Strophomenas but still distinctly visible. 



