92 OLAF HOLTEDAHL. M.-N. Kl. 



very difterent from Stroph. (?) Nefedjewi and Plectella. It does not seem 

 likely therefore that the Palaestrophonieiia type has developed out of the 

 latter; it is an indépendant type, with an exterior like a Stroplioniena 

 while the interior dorsal characters are like those in an Ortbis, among the 

 older representatives of which its precursors therefore must be sought. 

 To Eostrophonicna it shows some but no very great affinity. 



As I have not had an opportunity of knowing the interior of the other 

 resupinate forms occurring in 3 c ,j, I do not know if they must be con- 

 sidered as closely related to Palaeostrophomcna concava or not. However» 

 even if the interior characters of all these old forms from 3 c are rather 

 different from what is found in younger forms, there cannot be any doubt 

 that they represent the ancestors of the younger Strophomenids. 



I think we have here in 3 c the beginning of several evolutionary 

 lines, which we will now study somewhat more closely. 



As to the different paleontological characters which must be consi- 

 dered of especial importance for studies on evolution in tliese forms, — 

 consequently also for systematic studies, — I have reached the result that 

 not only interior characters but also general form and particularly the sur- 

 face features constitute relatively' good and stabile characters, t3'pical for 

 different lines of descent as I see them. 



The interesting form Palacostrophoiiicna concava shows a very striking 

 similarity in general form and surface ornamentation with a form of 3'ounger 

 age viz. Strophomena Broggcri form 4 b, the only difference in the exterior 

 being the somewhat larger size of the latter. 



I cannot see this fact otherwise than that, in spite of the difference in 

 the form of the cardinal process the younger form has developed out of 

 the older one. The stratigraphie horizon of the two forms is certainly so 

 different as to permit a modification, even rather great, of the cardinal 

 process, during that space of time. 



When proceeding into younger stages we do not meet a form i-e- 

 minding us of these two mentioned until the appearance of Aniphistrophia 

 Davidsoni. We here meet the same triangular outline, not very strongly 

 deflected margin and concentric corrugations. As to the interior of the 

 dorsal valve, there are some minor differences between this form and 

 Stroph. Brøggcri — the branches of the cardinal process having a less 

 elongate section, the crural plates more divergent anteriorly in the younger 

 form — these being too small to be of any importance for the question. 

 As will be mentioned farther down, I do not consider the denticulated 

 character of the hinge margin in the younger form to be a very impor- 

 tant svstematic feature. 



