1915- ^»«^- 12. THE 5TROPHOMEXIDAE OF THE KRISTIANIA REGION. 9I 



Evolutionary Evidences with Remarks on the 



Classification of the Strophomenidae. 



The oldest horizon in which we meet with Strophomenids in the 

 Kristiania region is the so called £".v^//5//^^-shale. 3 c ;^. In the basal Ordo- 

 vicic (the Ceratopyge division) in which in North America and also in 

 Sweden a representative of Walcott's genus Eostrophoniena has been found, 

 no Strophomenid in the restricted sense of the term has yet been traced. 



In 3 c i. however, they make a sudden appearance. We meet here a 

 fairly large number of forms, each representing a distinct type. Not less 

 than 6 forms are found, viz. the following i. Raßnfsqiiitia (?) sp., 2. Pa- 

 Ait'tistrop/ionitna concava Schmidt, 3. Strophoniena (?) Xefedjewi Eichw., 

 4. Strophomcna (?) Jenlzschi Gagel. 5. Strophoinena (?) cf. Aswiisi \*ern. 

 6. Plt'ctambonîtes (?J Kristianiat' n. sp. 



It is evident that the Strophomenid group already at the time of 3 c i. 

 was rather highly differentiated into various types. 



Wliile we do not know of any earlier Strophomenids in Norway, we 

 know in Russia already in the zones B I and B II (3 c ,i corresponds to 

 the lower part of B IIIi a considerable number of species belonging to the 

 genus Plectclla Lamaxskv. It seems very natural to regard the genus 

 Phctclla as the primitive type out of which at least some of the younger 

 Strophomenids have arisen. It is of very great interest that one of the 

 forms from 3 c p? (and from B III of the Baltic provinces) viz. Strophoniena (?) 

 Sefedjeivi shows in several characters much likeness to Plectclla. This like- 

 ness is very conspicuous in surface ornamentation and in interior charac- 

 ters, as can be seen in Russian specimens of S. (?) Xefedjcici. The outer 

 form differs, as the Plectellas are normally convex, whilst the younger 

 form is slightly resupinate. The size is equally small for both. 



This similarity between PlecteUa and 5. (?) Xefedjeici was mentioned 

 also by Lamanskv in his paper Die aeltesten silurischen Schichten 

 Russlands*, p. 159. 



Also 5. (?) Xefedjewi must be considered as a primitive type, a type 

 that has no very near relatives among the Strophomenids of either syn- 

 chronous or younger beds as for as is yet known. 



Of the other Strophomenids occurring in 3 c .i I do not know the 

 interior characters of any except the Palaeostroph. concava, which was lound 

 in the characters of the dorsal valve to be rather different from exteriorly 

 similar forms from younger beds. With its peculiar shape of the cardinal 

 process and nearly missing ridges in the muscular field it is certainly also 



