1915- ^O- !-• THE STROPHOMEXIDAE OF THE KRISTIANIA REGION. 83 



referred here. They are general!}' of smaller size than the typical speci- 

 mens and with the major ribs less apart. We may have relativelv Hat 

 specimens or very globose ones. 



In order to get information about the interior of the valves I 

 treated the dorsal valve (that is of most interest) of a free specimen with 

 acid and thus obtained some idea of the structure. We observe a number 

 of subparallel grooves, corresponding to the ridges of the muscular area 

 of the valve; in the median line a rather short, not prominent groove, on 

 both sides a ver\- long one, slightly diverging towards the front, further 

 a pair of shorter grooves, and possibly still a third faint pair further 

 outwards. 



Observations. The internal structure of PI. Scluuidti seems to 

 be of about the same kind as in P. transversal is, and somewhat different 

 from that found in P. scncea. Specimens of P. transversalis, however, 

 that have been treated in the same manner, show much broader and more 

 prominent grooves, probably indicating coarser ridges in the valve. 



Occurrence. 5 a — b. Stavnæstangen, Vestre Svartø, Østre Svarto, 

 Ringerike. Elspecially common in 5 a. 



Plectambonites transversalis Wahl. 

 (PI. XV, fig. 1—4.) 



For European S3-nonom\- see Lfpioena iraitsversalis Wahl in Davidson, Brit. Sil. 



Brach, p. 318. 

 For American S\nonom\' see Plectatubottites transversalis Wahl in Schcchert, 



Synopsis, p. 311. 



This well known and often cited specimen shows in the Kristiania 

 region as elsewhere an exeedingh- large variation in size and form. However, 

 though extending throughout a very considerable space of time, no distinc 

 variation can be fixed for different stratigraphie horizons, considerable 

 variations being found among the specimens occurring in each stage. 



The majorit}' of specimens are found in stage 6, where also the 

 largest and prettiest types are met with (see pi. X\', fig. 2). Besides this 

 large type another much smaller, very strongl}- convex and relatively longer 

 (in outline much like the specimen from which Dalman's original figures 

 were drawn) is met with, see pi. XV, fig. i. In stages 7 and 8 P. trans- 

 versalis is more rarely found, however, also occurring there in var\Mng 

 shapes. The specimens from 8 are generally rather small, not seldom 

 very broad. The latter is the type that Davidson in 1847 called Leptcena 

 Duvalii, which however in his large monograph he includes under L. trans- 

 versalis. 



