154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 6, 



three times before they reach the margin, where they are between 70 

 and 80 in number : the ribs are more closely crowded together at 

 the sides than on the front of the shell. 



Hinge-line nearly of the extreme width of the shell, with its 

 angles sUghtly rounded off. 



Length ^ an inch, breadth f of an inch. 



Found in both divisions of the Lower Silurian beds in the Serra 

 de Bussaco. 



Only one valve is figured, as the impressions of the two valves 

 cannot be distinguished. The character by which the species is most 

 easily recognized is the crowding of the side ribs contrasted with the 

 looser ribbing of the front of the shell. 



Orthis Mund^, n. s. Pl. VIIL fig. 5. 



O. testa tmncato-elliptica, convexiuscula, costata; costis numerosis ro- 

 tundatis inaequalibus, lateralibus arcuatis ; lineis transversis coucentricis 

 creberrimis. 



Shell truncato-elhptical, slightly convex, ornamented with nume- 

 rous narrow rounded unequal ribs all springing from the umbo, in- 

 creasing in number towards the margin, and crossed by numerous 

 concentric lines ; the side ribs are curved : end of the hmge-line 

 slightly rounded off : shell thin, with the ribbing distinctly marked 

 on the interior. 



Length y^o^ths, breadth 1 inch. 



From the upper division of the Lower Silurian formation at the 

 Porto de Santa Anna, in the Serra de Bussaco. 



Plate VIIL fig. 5 a. Dorsal valve, exterior. 



Fig. 5 b. Dorsal valve, interior. 



Orthis Berthoisi, Rouault, Bull. Soc. Geol. Fr. 2 Ser. vol. vi. 

 t. 2. f. 4. Pl. VIIL fig. 4. 



O. testa ovata, utrinque convexiuscula, striata ; valva ventrali majore ad 

 apieem depressiuscula ; striis numerosissimis subsequalibus dichotomis, 

 mediis rectis, lateralibus arcuatis ; lineis concentricis raris. 



Shell ovate, with a very short hinge-line ; dorsal valve slightly 

 convex ; ventral valve more convex than the dorsal, with a slight 

 mesial depression, which beginning at the umbo dies out before 

 reaching the middle of the valve ; both valves covered with very 

 numerous fine bifurcating ribs, which divide many times before they 

 reach the margin, and are crossed at irregular distances by strong 

 concentric lines ; lateral ribs curved. 



Length 1 inch ; greatest breadth below the middle, f of an inch. 



Abundant in the upper division of the Lower Silurian formation 

 at the Porto de Santa Aima, in the Serra de Bussaco. 



In general form and external markings this shell closely resembles 

 the common Carboniferous species Orthis Michelini of Leveille {0. 

 filiaria, Phill.) ; the only obvious distinction being the slight de- 

 pression near the umbo of the deeper valve ; but the form of muscular 

 impressions of the dorsal valve affords a sufiicient distinction. 



