I02 



OLAF HOLTEDAHL, M.-N. Kl, 



forms, Stropheodonta and Strophonella, but also in the older types, Rafines- 

 quina and Strophomena. However, as unfortunately I know very little of 

 the dorsal interiors of the Rafinesquinas of the present material I shall 

 not enter into this question. For the same reason I cannot tell if the 

 Norwegian Rafinesquinas are all to be regarded as having developed out 

 of types with the bilobed cardinal process and the prominent ridges in the 

 dorsal interior as characterise Raf. imbrex — and the Strophomenas out of 

 forms like Palaeostrophomcna concava — or whether the question is more 

 complicated. 



When regarding as a whole the Strophomenids at their full develop- 

 ment in the upper Ordovicic and the Siluric we get the following impres- 

 sion: we have before as a great number of types, the majority of which, 

 however, are mutually strongly related, while besides these we find a few 

 others greatly different from the rest and also mutually different. The first 

 mentioned larger group consists of the ordinary fairly large Strophomenids, 

 including the genera Rafinesquina, Stroplieodotita, Strophomena, Stropho- 

 nella, Schuchertella and Lcptacna. Even if these forms have their origin 

 along somewhat different lines, I do not think it can be questioned that 

 they all are fairly closely related. Besides their many similar exterior fea- 

 tures, their internal characters, both in ventral and dorsal valves, are also 

 relatively very corresponding. The character of their muscular impres- 

 sions, of the cardinal process and crural plates show a great, often ex- 

 ceedingly great likeness. 



Opposite to this large group, we have the more isolated small ones, 

 we have Plectambonites, we have Christiania and we have the group 

 Triplecia ^Streptis, all with peculiar external as well as internal characters. 



According to the writer's opinion therefore the classification that 

 Hall and Clarke gave at the end of their »Introduction« Vol. II, where 

 Strophomena is placed in one family (also including Triplecia); Leptœna, 

 Rafinesquina, Stropheodonta and Strophonella in an other, the latter including 

 Plectambonites and Christiania, is far from being natural, Schuchert, in 

 his classifications of 1893 (American Geologist) and 1897 (Synopsis of Am. 

 Foss, Brach.) has retained the two groups of Hall and Clarke, but 

 includes both — as two ^z^Äfamilies Orthothetinœ and Rafmesquininœ — 

 in one family the Strophomenidce. In his newest classification, however ^ 

 Schuchert has done what seems to the writer to be the correct thing, 

 he has for some of these minor groups of Strophomenids established 

 special subfamilies thus emphasizing their indépendant systematic value. 



1 In Eastmans Edition of Zittel's Text- book of Paleontology, 1913. 



