516 



i* i ■ i.i etin: museum »»i < ompara i i\ e zo< <\ 



main sea. The lift entrapped in these basins must have been nib- 

 jected to n-v. environmental conditions. A change in the chemical 

 con t itution resulting from a freshening of the sea-water due to the 

 new drainage; a concentration of Bait; an abnormal increase in 

 hydrogen sulphide and other gases; mud and other impurities; varia- 

 tions in the temperature and the depth of water are agencies in 

 dwarfing according to Professor Shinier. 



It seems that in the case of the Strophomenae and rXafinesquinae 

 from Clermont, the ingress of mud was probably the determining 

 factor. I pon examination of the fine-grained shales that contain 

 these fossils, it is easy to imagine that conditions during the Maquo- 

 keta deposition in this region were not unlike (hose that exist today 

 in the Mediterranean Sea. De Lapparent noticed that while the 

 fauna of the western Mediterranean was dwarfed, that of the eastern 

 part was still further affected. He attributed this to the presence in 

 the water of the eastern basin of fine particles of mud, which sank to 

 the bottom only very slowly. Fine grains of mud taken into the 

 tender soft parts of these brachiopods might easily have caused an 

 irritation that would result in the prevention of normal growth. 



Rafinesquina altidorsata, sp. nov. 

 Plate 2, fig. 2. 



Shell small and highly convex, almost hemispherical, with the 

 greatest convexity in mature specimens a little nearer the beak than 

 the anterior margin, although very nearly mid-way between. 



The more prominent radiating lines of the pedicle-valve occur at 

 intervals of less than one millimeter and the intermediate spaces are 

 occupied by four to six finer striae. 



The interior of the brachial-valve is thickened along the anterior 

 margin. Three ridges running lengthwise to the hinge embrace the 

 four adductor muscle-scars. 



Typical specimens measured: — 



V, idth Length 



22 mm. 19 mm. 



24.5 mm. 19 mm. 



26 mm. 19.5 mm. 



Greatest convexity 



11 mm. 

 10 mm. 

 13.5 mm. 



The common Maquoketa representative of the genus at Delafield, 

 ^Yisconsin and Spring Valley, Minnesota, is R. kingi (TVhitfield), but 

 its place was taken at Clermont by -two new forms, R. subquadrata and 



