I9I5- ^C>- !-■ THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 93 



A form like the Strophomena (?) Jentsschi, Gagel from 3 c ver}' pro- 

 bably is the precursor of S. psendodeltoidea Stollev from 5. The outer 

 moulds of ventral valves that are figured pi. VI, fig. i and 2 give a good 

 impression of their strikingly similar appearence. Both general form and 

 surface character strongly correspond, the size, however, being larger 

 in the younger form. 



The single, strongly concave specimen (S. cf. pseiidodelto'uica) from 6 a, 

 Hvalsbakken, Asker, certainly belongs to the same line. 



The fragmentary very poorly preserved ventral \ alve from 3 c;:?, de- 

 signated Strophomena (?) c\. Asmiisi \'ern. may, with much probability be 

 considered as the oldest stage known of the Strophomena Asmiisi line. 

 The representatives of this group of Strophomena Asmiisi are among those 

 most commonly met with in the upper part of the Ordovicic of the Kri- 

 stiania region, occurring from the lower part of the 4 b to the top ot 5 

 and probably reaching into stage 6. As was stated under the description 

 of S. Asmiisi we meet with a great variation in the said species, the older 

 types being larger, and with their major radiating ribs at greater intervals 

 than in younger ones. 



Strophomena Kicrri from 5 that in size and general form strongly 

 reminds one of 5. Asmasi is certainl}- not ver\- closely related to this 

 species, as both interior characters and surface features differ considerably. 



The Strophomena n'gida Barr, lives through 4 b and 4 c and in 4 d is 

 represented by the variety frogfioensis. Further it seems extremely probably 

 that this line is continued by Stroph. antiqiiata (in the type of shell for 

 which this name is here used), extending trough the larger part of the siluric 

 <6— 8|. Both size, general form, and especially the characteristic surface orna- 

 mentation, with a number of ver\' fine radiating lines between stronger ones, 

 and the intervals between the latter with the typical undulating surface, 

 correspond very well. I have in fact before me a specimen from stage 8 

 that in exterior characters can scarcely be separated from one from 4 c. 

 Also the tiny little Stroph. Loveni Vern. belongs certainly to this group, 

 its chiefly differing character being the small size. 



Another parallel line, of somewhat less vertical range, is character- 

 ised by fairly small, slightly sinuated Strophomenas, with coarse and equal 

 radiating striae. We have here S. Scofieldi W. and S. from 4 b, Stroph. 

 sp. (p. 61) from 4 c and S. costatula Hall and Clarke from t. 



Of the normally convex forms, the Rafinesqiiinas , the first represen- 

 tatives are met in the lower part of 4 b if we do not consider the Rafiues- 

 quina (?) sp. from 3 c. At the base of 4 b we meet R. imôrex Pand. a form 

 that is found in still older beds in Russia (in B llll. With the very pro- 



