70 OLAF HOLTEDAHL. M.-N. Ki. 



specimens like the one depicted in pi. XII, fig, 6, but we have also forms 

 with the central disc longer (see pi. XII, fig. 4), more semicircular, though 

 always with well developed cardinal extremities. Most of the specimens 

 from 4 b have the striae of somewhat different size, a small number — especi- 

 all}^ in the median portion — being stronger than the rest, but in some speci- 

 mens no such difference can be seen. As to the size, these oldest specimens 

 generally fairly small, the largest being 4,2 cm. wide at hinge, length 

 of central disc about 2 cm. 



In one ventral valve, where the shell substance has been removed, 

 the mould shows a muscular field, rather faintly marked, no indication of a 

 bordering ridge being seen in front. No well defined adductor scars are 

 seen, only a faint median line. 



From 4 c and 4 d the material is rather poor, it is however evident 

 that e. g. the outline also varies verv strongly here. 



Stange 5. A very large material is present from 5. Though vary- 

 ing somewhat, the specimens generally seem to have certain features in 

 common which distinguish them from the older types. They nearly always 

 show: more subrectangular outline of the central disc, a distinct ridge- 

 like elevation surrounding that disc, making the geniculation extremely 

 sharp, a coarser radiating striation, the striae being in most (not in all 

 cases") of uniform size, and large expanded lateral wings. Concerning the 

 size, there does not seem to be' any difterence between specimens from 4 

 and 5. The muscular field in a ventral valve was found to be more com- 

 pletely surrounded by an elevated ridge than in the specimen from 4 b. 

 The mould in question is not distinct enough to judge as to the adduc- 

 tor .scars. 



Stuges 6 — 7. Figs 3, 7. The specimens belonging here are of 

 exactly the same type as in 5. Here too a number of the radiating striae in 

 the median portion of the valves may be somewhat broader and stronger 

 than the rest. 



Stage 8. Figs. 2, 5, 10, 11. We have here a much larger variety in 

 the external appearance of L. rhomboidalis than in stages 5 — 7. First we 

 have the type we meet in these older divisions. A further development 

 in size of this type is represented by some extremely large specimens that are 

 characteristic of the zone 8 c of the Bundefjord district. These sometimes 

 reach a width at hinge line of as much as 6 cm., a length of central disc of 3. 

 The lateral margins of the central disc of this large type usually diverge con- 

 siderably towards the hinge, giving a less rectangular oudine than in the 



