BIHANG TILL K. SV. VET.-AKAD. UANDL. BAND. 3. N:0 12. 13 



Length. Breadth. 



Dorsal valve 8 mm. 11 mm. 



» » (j )^ 9 » 



Thoiigh I liave coUected thousands of specimens of this 

 species, I have only found a very few tolerably good ones. The 

 internal casts are generally the best, but even tliese are nearly 

 always defective or obsciire in the cardinal region. 1 have had 

 some casts figured. Figs. 14 and 15 represent typical forms of the 

 internal cast of the ventral valve; fig. 16 represents a specimen 

 somewhat deviating through the radiating ribs, which are un- 

 iisually coarse and extend to the cardinal region. In the ligured 

 internal cast of the dorsal valve (fig. 19) the lobes corresponding 

 to the grooves in the interiör between the cardinal process and 

 the brachial plates are larger than usual. 



Among previously described species none, perhaps, is more 

 nearly related to this than O. Hicksi (Salt.) Dav. The latter, 

 however, differs by having fewer and coarser ribs. It also seeins 

 to have the bbak less pointed, and the area lower than O. e,r- 

 j^orrecta. 



O. exporrecta occurs chiefly in the strata with Paradoxides 

 Forchhammeri. In Westrogothia it is by far the most abundant 

 fossil of that horizon, and it is very plentiful wherever these 

 strata are accessible, as at Kinnekulle, Lovened, and Gudhem. I 

 have also found it in several places in Nerike; at Södra Möck- 

 leby in Öland; and, in loose stones, at Lillviken in Jemtland. 

 In the strata with Agnostus laevigatus I have found some few 

 specimens at Carlsfors in Westrogothia, near the north end ot 

 mount Billingen. At Andrarum in Scania and at Bornholm, 

 where the strata with Paradoxides Forchhammeri contain a very 

 rich fauna, this species, as well as the preceding one, seems to 

 be altogether wanting. 



Orthis Hicksi (Salt.) Dav. aif. 



Pl. III, figs. 22, 23. 



By the above name I will designate, provisionally, a species 

 of which hitherto, besides a single interiör of the dorsal valve, 

 only a very few, mostly incomplete, internal casts have been 

 found, and for which, therefore, I will not create a new specific 

 name. It is nearly related to the preceding, but perhaps still 

 more so to the British O. Hicksi, though I do not think it im- 



