88 OLAF HOLTEDAHL. M.-N. Kl. 



Sinus and fold not taken into consideration the convexit}' of ventral and 

 dorsal valve is about equal or the dorsal somewhat more convex. Sinus 

 and fold in adult specimens very strongly developed, varying somewhat 

 in different specimens. Width of sinus and fold at the frontal margin 

 amounts generally to between i^ and V2 of the total width of the shell. 

 While in adult specimens sinus and fold are bounded by distinctly marked 

 nearly vertical sides, these are missing in quite small specimens, the broad 

 shallow sinus and the slighdy elevated fold passing more gradually into 

 the rest of the surface. The sinus and fold are as seen from the front 

 of the shell quite commonly not vertically but obliquely directed, the front 

 in these cases having an assymetric aspect. 



The cardinal area of the ventral valve is low, the deltidium very 

 narrow. No foramen at the beak is visible. 



The surface characters are those characteristic of the gei/iis Streptis: 

 a number of delicate concentrically arranged ridges, each marked as can 

 be seen in good preservation by a row of small tubercles, the detailed 

 form and character of which I have not been able to study in any speci- 

 men. The number of concentric ridges in very small specimens is about 

 10, in adult ones generally between 15 and 20. 



Observations. This interesting little form bears a strong resem- 

 blance to Streptis monilifera M'CoY ^ from the British Caradoc and Llandovery 

 but can be separated by some differing characters, none of which, however, 

 are of great systematic importance. As is indicated by the varietal name pro- 

 posed for the Norwegian form, the latter is characterized by a much more 

 developed sinus (and fold), this being in the British species rather faint. 

 It is interesting to note that the typical Streptis monilifera in this feature . 

 strongly reminds us of the immature specimens of the Norwegian variety, 

 the specimens depicted by Davidson being also smalfer than the adult 

 Norwegian ones. Another and quite striking difference is the far larger 

 number of concentric ridges in the latter, here amounting to 15—20, while 

 for the British form both in text and illustration it is stated as being 8—9. 

 Also in this respect the very small Norwegian specimens show a greater 

 likeness to the British form than the adult ones. 



Occurrence 6a — b. Konglungen and Spirodden, Asker. 



1 See Producta monilifera M'Coy. Synopsis Sil. Foss. Irai. p. 25, pi. Ill, fig. 3. 



Triplesia (?) monilifera Davidson. Brit. Sil. Brach, p. 200, pi. XXV. fig. 3- 

 Triplesiail) monilifera Dav. Sil. Siippl. p. 147, pi. VIII, fig. 31. 



