1915- '^O- 12. THE STROPHOMEXIDAE OF THE KRISTIANIA REGION. 95 



KiESOW from 4 b, as far as can be Judged from the fragment tnat is 

 found. 



As to the long living Lcptacna rhomboidalis , that begins in 4 b, the 

 different var\'ing types of that form are mentioned above. The oldest ones 

 with their less sharply defined vertral muscular field than in high siluriC 

 specimens show here a feature, that reminds us of Rafinesqitina, pointing 

 towards a common origin of this genus and Lcptacna. The older type 

 has also commonly a somewhat differing outline compared with the younger 

 ones, the central, disc being less rectangular, more subtriangular or roun- 

 ded also in this feature reminding us of Rafiuesqnina. The richest develop- 

 ment of Lcpt. rhoiuboidalis with most variation is found in stages 8 — 9 

 where also the largest sizes are observed. 



Of Schitcliertellas onh' a single species, the well knovn Sclmchcrtclla 

 pectcii Dalman has been i dentified. This has, however, a very consider- 

 able vertical range, passing from stage 5 and through 8, showing in 

 this space of time much variation in size but not in other characters. 

 The few fragmentary specimens that are known from stage 5 are verv 

 small, and the bulk of specimens from 6 are also tairly small. In the 

 upper part of 8 in the Bundefjord district S. pcctcii attain to a 

 conparatively huge size, and at the same time show a great deal of 

 individual variation. As to the question as to from what sort of older 

 Strophomenids 5. pcctcii has developed it seemed at first natural to think 

 of a type like Strophouioia AsDinsi that has a verv similar outer form. 

 A fact that at an earh' date of this investigation I thought pointed in this 

 direction, was that the radiating ribs of specimens of Sfrop/ionicna Asmnsi 

 from younger beds (stages 5 and 6) as a rule were of more uniform size, 

 i. e. more like those of Sclmchcrtella pcctcii, than in the older t\-pe Trom 

 4 b etc.). However, having worked out the interior characters of a number 

 of dorsal valves of the two species from different stages it became evi- 

 dent that they had but little to do with each other. The interior of 

 Stropb. Asmiisi was exacth' of the same t3^pe in specimens from 5 as in 

 those from 4 b and the same constant character in the dorsal interior was 

 found in Sch. pectcn from 6 and from 8. Specimens of the latter from 

 both stages had the characteristic short, but very prominent, straight, and 

 abruptly ending crural plates, highly different from the long delicate, 

 curved, gradually decreasing ones in Stropli. Asmnsi. They are much 

 more like those in another, onl}' slighl}»^ concave Strophonieiia, viz. 

 S. Kiœri from 5 and, as far as can be judged from what has been 

 observed in indistinctly preserved interiors, still more like those of S. rigida 

 Barr, from 4 b — c. 



