152 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 29, 



OrTHIS MiNlENSIS, n. sp. 



Shell rather flat ; outluie nearly semicircular, with the greatest 

 width at the hinge. 



Dorsal valve moderately convex, with a slight mesial elevation. 



Ventral valve flat, with a faint mesial depression. 



Valves covered with well-marked concentric lines and ornamented 

 with 20 to 30 narrow rounded simple ribs, reaching from the hinge- 

 area to the margin, and separated by broad flat interstices. At the 

 middle of each valve short narrower ribs are inserted between the 

 others, giving that part a more crowded appearance than the sides of 

 the shell : these intermediate ribs vary from 4 to 8. 



Length |- of an inch ; width 1^ inch. 



Found in slate near Vallongo. 



Fig. 3 a. PI. VI. Dorsal valve, from the cast of a fragment in which 

 the shell is rendered narrower by compression. 



Fig. 3 h. Dorsal valve, from a cast : in this specimen the width of 

 the shell is exaggerated. 



Orthis Duriensis, n. sp. 



Shell concavo-convex ; outline nearly semicircular ; greatest width 

 close below the hinge. 



Ventral valve slightly concave, with a broad, shallow mesial sinus ; 

 ornamented \^dth about 50 delicate, slightly raised ribs, all reaching 

 from the hinge to the margin, and separated by wide interstices. Be- 

 fore they reach the middle of the valve a narrow furrow is impressed 

 upon each rib, this grows a little wider and deeper, and finally divides 

 the rib into two faint rays, which run on together to the margin, 

 which they reach in regular pairs. Hinge-area very narrow. 



Dorsal valve ; hinge-area broad and longitudinally striated. An- 

 other specimen, which appears to be the dorsal valve of this species, 

 is regularly convex, without any mesial elevation. 



Length \ of an inch ; mdth If inch. 



Found in slate at Vallongo. 



Fig. 4 a. Ventral valve, from a cast. 



Fig. 4 h. Ribs of the same specimen magnified. 



Orthis Lusitanica, n. sp. 



Shell concavo-convex ; outline rather exceeding a semicircle, with 

 the angles at the end of the hinge-line rounded ofl" ; greatest width 

 a little below the hinge. 



Dorsal valve regularly convex, the greatest elevation being about 

 the middle of the valve ; hinge-area twice the breadth of that of the 

 ventral valve. 



Ventral valve somewhat concave ; hinge-area narrow. 



Valves covered with fine concentric lines, and ornamented with 24 

 to 30 narrow rounded simple equal ribs, all reaching from the hinge- 

 line to the margin, and separated by broad flattened interstices. 



Length \ of an inch ; width at the hinge 1^ inch ; greatest width 

 If inch. 



