1915- No. 12. THE STROPHOMEXIDAE OF THE KRISTJAXIA REGION. lOI 



tion is found in a form with convex ventral vahre, also occimring in the 

 middle part of the Ordovicic, Raf. Mfiitsttri. Another somewhat similar 

 varietj- of surface marking, with the radiating ribs nearly invbible in the 

 umbonal region, but with the said undulations, here more like small, fairly 

 well rounded depressions, is seen in a Strophomena. S. loveni and in two 

 forms with convex ventral valve Raf I?) cf. r/egatis from 4 and Raßiies- 

 quina (?J sp. from stage 8. In stage 5 occurs a slightly resupinate form 

 with verj- conspicuous wrinkles at the hinge, but not elsewhere. S. Kiœri. 

 at the same time there is a Rafinesquina with the same characteristic fea- 

 ture. R. pseMiioaUtmata ScHMrøT. A subtriangular outline, deflected mar- 

 gin and distinct concentric wrinkles are found in the resupinate forms 

 StrophometHM Broggeri and Amphistr»phia Davitlsoin, it occurs also in a 

 form with normal convexity Raf l?l Schnatfti. Still many similar exam- 

 ples could be mentioned, showing particular external characters occurring 

 in both groups of shells. 



However, as an isolated, highly interesting circumstance must be men- 

 tioned what is described in greater detail on p. 50. that among a very 

 large number of resupinate specimens of Strophomena Braggrn a single 

 one with distinctly convex ventral and concave dorsal valve is tbund. 

 This fact shows better than any other that the resupination of the valves 

 cannot always be relied on as a systematic character. 



As to the interior characters we in most cases have a pecuUar feature 

 in the resupinate shells, that makes them diiffer from the normally convex 

 forms in one respect : the former have their ventral muscular field generally 

 better defined, bordered also in front by a distinct, surrounding ridge, 

 commonly faintly developed or missing except in the posterior part of the 

 muscular area in convex ventral valves. However, I do not think this 

 character to be of great systematic importance. I think it might very well 

 be considered as a secondary character only, the ridges at front being de- 

 veloped — for the sake of a better attachment of the lai^e diductor mus- 

 cles — only in the concave lor flat) valves, just because they are con- 

 cave, the muscular field thus becoming in both convex and concave ven- 

 tral valves of a somewhat similar tj-pe: concave with elevated margins. 

 Minor differences in the development of the surrounding ridges of the mus- 

 cular area may of course be due also to other causes, a thick shdied 

 form having for instance relatively strong ridges etc. 



As to the characters that I consider of most systematic importance, 

 viz. the interior features of the dorsal valves, also these seem in many 

 cases, 35 can be judged from existing illustrations, to be rather like in 

 normally curved and resupinate Strophomenids. not only in the younger 



