1915- ^O. 12. THE STROPHOMENIDAE OF THE KRISTiAmA REGION. 63 



Observations. This form, that seems to correspond very well 

 with the American Niagara form mentioned, comes except in size close to 

 one of the types that Davidson has included under Stroph. antiquala, viz. 

 the type (see figs. lo and ii in pi XL1\" of his monograph) with closely 

 arranged radiating ribs. 



Occurrence. 6a, Høierholmen and Spirodden, Asker. 



Strophomena sp. 

 (PI. XIII. fig. 8.) 



A single medium sized, rather concave ventral valve cannot be 

 referred to any of the said species but shows, however, a good deal ot 

 likeness to several. The radiating ribs are rather coarse, of many different 

 sizes, irregularly arranged. No distinct sinus is developed. 



The specimen reminds somewhat of both StropJiomena atiiiqtiata (in 

 the sense used here) and 5. costatula, in surface markings coming be- 

 tween both. 



Occurrence. 6c, Malmo. Bundefjord. 



Genus Strophonella Hall 1879. 

 Subgenus Ampbistrophia Hall and Clarke 1892. 



In p. 292 of their »Introduction« Hall and Clarke, discussing the 

 oldest known American representatives of Strophonella viz. 5. patenta and 

 S. striata write: »Should it be considered useful to recognize the incipient 

 and progressive feature of the species S- striata, and probably S. patenta, 

 towards a full manifestation of generic characters, and distinguish them 

 from Strophonella in its more mature condition of development, the term 

 Amphistrophia may prove expressive of their apparent double relation as 

 shown in the young and mature shells.« Of 5. striata is written in the 

 same place: y Strophonella striata. Hall, of the Niagara fauna, has a delti- 

 dium highly developed in the immature stages of the shell, a prominent 

 deltidial callus, and a short row of cardinal denticulations ; it is, in fact, 

 in these respects a reversed Brachyprion, bearing precisely the same rela- 

 tion to Strophonella in its fuller development as that group does to 

 Stropheodonta. < 



There are in the Norwegian material two forms which naturally come 

 into Hall and Clarkes group or subgenus Amphistrophia, as the}- must 

 be considered as precursory types of true Strophonellas. Even if not all 

 of their internal characters are known, one of especial interest has been 



