I915 No. 12 THE STROPHOMEXIDAE OF THE KRISTIANIA REGION. 37 



feature places the form under the genus Stropheodonta or, strictly speaking 

 under Bracftyprion Shaler. the precursor of the typical Stropheodonta. 



As regards interior characters, the ventral muscular area can be 

 nicely seen in several specimens. It is well marked posteriorly, its straight 

 borders here directed at about right angles, sometimes more, sometimes 

 less, not meeting, however, but reaching the hinge at a distance of from 

 2 to 4 mm. The adductor scars that are irregularly longitudinal!}' 

 striated are usualh- well marked Isee pi. W , fig. 71. Anteriorly the rather 

 large diductors are indistinctly marked. The lobated margin can be traced 

 in finely preserved specimens because of the different surface characters 

 of scars and the rest of the interior of the valve, the latter being punctate. 

 In a dorsal valve showing parts of the interior, a well developed bilobed 

 cardinal process can be seen, the muscular impression being indistinctly 

 limited. 



Observations. The form here described being without doubt 

 extremely nearly related to Strophomena imbrex Pand. var. semiglobosa 

 Davidson ^, but at the same time shows so much of differences that it 

 cannot be placed under that form. The difference lies especially in outline 

 and convexity. In Davidson's species a typical feature is the relative 

 narrowness of the shell at the hinge, the outline broadening distinctly 

 anteriorly, the greatest width being at about the middle of the shell. In 

 the Norwegian specimens the rule is undoubtedly that the greatest 

 width is at the hinge line, the lateral margins from here converging 

 anteriorly. Another distinguishing feature is the different convexit}', the 

 British form being generally more convex than the Norwegian. These two 

 characters — ver\- strongly expressed convexit}- and greatest width at 

 about the middle of the length of the shell — give to Stropheodonta - f Bra- 

 cftyprion) semiglobosa that globose form, that is expressed in DA\nDsoN'5 

 name, features that are not at all so strikingly developed in the Norwe- 

 gian specimens. To judge from existing illustrations also the size is 

 different, the British specimens being larger. 



Occurrence. The great majority of the specimens are from 8 d, 

 Malmø in Bundetjord and from 9 d, Isl. off Holmestrand; rather similar 

 specimens occur also in 7 in different districts. 



1 See Brit. Sil. Brach, p. 286, pi. XLI, figs. 1—6- 



^ In Davidsox's Sgures showing the dorsal cardinal area a distinct denticulation on the 

 inner margin is marked. 



