76 OLAF HOLTEDAHL. M.-N. Kl. 



now used for a considerable time in the huge American literature, and 

 also elsewhere. A change here would create more confusion in the 

 terminology than already exists regarding the Strophomenids. 



Plectambonites(^) Kristiania,e n. sp. 

 (PI. XVI, fig. 2.) 



Mat. pres. A moderately large number of ventral valves (or 

 specimens with only the ventral valve exposed) in rock. 



Description. Shell of very small size, the largest specimen seen 

 measuring in width 7 mm., in length slightly more than 3. Outline broad, 

 subsemicircular with very sharp, projected cardinal angles. Ratio width: 

 length generally about 2:1, often the width is relatively still larger. Con- 

 vexity rather great in longitudinal section, especially in the umbonal 

 region. We have here a triangular portion of the valve that is distinctly 

 elevated above the flatter lateral parts. The posterior angle of this 

 umbonal elevated portion varies from 60 to 80°. This triangular median 

 portion is made still more conspicuous by two ver}' strong radiating ribs 

 just bordering it. These two ribs are both broader and in particular much 

 more elevated than the rest of the radiating striae that are fairly broad, 

 but ver}' low, and therefore slightly prominent even where the surface is 

 well preserved. The number of ribs between the two strong ones, counting 

 near the front, is in fairly large sized specimens 10 or somewhat more. 

 Because of the flatness of the ribs the exact number is difficult to deter- 

 mine. Near the hinge line the ribs are still more indistinct, but the surface 

 is here crossed by a small number (about 4I of parallel wrinkles, pointing 

 strongly outwards, following the direction of the lateral portion of the 

 rounded margin. These wrinkles gradually disappear just before reaching 

 the two strong, diverging ribs. A very faint indication of concentric 

 wrinkles may also be seen in the median portion between the two ribs. 



Observations. Unfortunately I do not know the interior charac- 

 ters of this form and its generic position therefore is not quite certain. 

 Although strongly resembling Plectambonites in general appearence, the 

 interior of this very old form may be difi^erent from what is formed in 

 the typical Plectambonites species from younger beds. Of Norwegian 

 Plectambonites forms the relatively coarsely striated P. cf papulosa Cowper 

 Reed, mentioned farther down, come nearest in general appearence, but 

 also differs in several respects. In fact the P. (?) Kristiaiiiae have a very 

 peculiar aspect, given by the two strong, greatly diverging ribs, conspi- 

 cuous among the rest, and by the wrinkles near the hinge, these wrinkles 



