36 



OLAF HOLTEDAHL. 



M-N. Kl. 



Brachyprion semiglobosum Dav. var. norvegicam n. var. 

 (PI. IV, fig. 6-9.) 



Mat. pres. A large number of mostly badly preserved ventral 

 valves, some outer moulds of dorsal valves, all in rock, and a few frag- 

 ments of free specimens. 



Description. Shell of medium size, not varying much. The 

 exact outline can generally not be seen distinctly as the cardinal angles in 

 nearly all specimens are somewat broken, but may in most instances be 

 judged from what is seen of the rest of the shell. The outline is nicely 

 and broadly rounded in front, the margins becoming gradually more 

 parallel towards the hinge, meeting the latter at an angle of about 90 ^ or 

 somewhat less, not taking into consideration a small outward curving at 

 the very hinge line. The greatest width of the shell is thus generally at 

 the hinge, possibly with a few exceptions. 



The width and length of a few specimens are given below in mm. : 

 Width 38 43 45 

 Length 29 32 39 



The convexity varies considerably, and is generally rather strong. The 

 lines in the figure in the text (beak to the left) will give an impression of the 



curvature of ventral as well as dorsal valves, in a 

 longitudinal direction. The ventral valves usu- 

 ally have a fairly even and regular convexity, 

 the greatest elevation lying somewhat behind 

 the centre of the valve. We may also have a 

 more geniculated curvature, with a relatively 

 flat central disc from the hinge and some distance 

 forwards The dorsal valves are considerably 

 less curved than the ventral. 



Surface markings are not well preserved in 

 any specimen, but certain chief features may be 

 Lines showing curving in median Seen in some. The radiating ribs are of two 

 line of Brachyprion semiglobo- sizes, the intervals between the stronger ones 

 sum Av. var. norvegicuni e j-gi^j-jyeiy small, generally under i mm., and 



5 upper represent ventral, the "^ »^ ^ 



3 lower dorsal valves. the number of fine striæ in each correspondingly 



small, as it seems mostly 2 — 3. Of concentric corrugations no indication 

 is seen. 



The cardinal area can be pardy seen in two ventral valves. It is 

 not very well preserved, but on the inner margin a series of denticulations 

 can be seen, reaching from the delthyrium about Vo cm. laterally. This 



