1853.] RTBEIRO AND SHARPE — BUSSACO. 153 



very numerous, fine, dichotomising ribs which all radiate from the 

 apex to the margin. 



Dorsal valve elevated along the middle, and depressed tpwards the 

 side margins. 



Ventral valve depressed down the middle, and elevated towards the 

 side margins, having thus an undulating surface. Hinge-area narrow, 

 produced at the extremities. 



This very elegant Orthis differs from all the species yet figured, 

 in combining the outline of a Spirifer with produced angles with 

 fine dividing rays all springing from the apex. 



The English species most nearly allied to it is the Spirifer alatus 

 of the Sil. Syst., which Mr. Morris has properly transferred to the 

 genus Orthis. 



I have named this species after Senhor Carlos Ribeiro, to whom 

 we owe the information contained in the present paper. 



Length | of an inch, breadth at the hinge 1§ inch. 



Common in the Lower Silurian rocks of the Serra de Mucela, east 

 of Coimbra. 



Plate VIII. fig. 1 a. Dorsal valve, exterior. 



Fig. 1 b. Dorsal valve, mterior. 



Fig. 1 c. Ventral valve, exterior. 



Fig. 1 d. Ventral valve, interior. 



Orthis exornata, n. s. Pl. VIII. fig. 2. 

 O. testa subrotunda, plano-convexa, costata ; costis 7-8 elevatissimis acutis, 

 bis trichotomis. 



Shell plano-convex, with a rounded outline ; dorsal valve very 

 convex, ornamented with seven groups of highly elevated sharp ribs ; 

 each group consists of a single rib starting from the umbo, and soon 

 dividing into three, of which the middle rib is the largest ; each of 

 these ribs divides again into two or three near the margin. 



Ventral valve nearly flat, with eight groups of ribs less elevated 

 but similar in character to those of the other valve. 



Hinge-line rather less than the greatest width of the shell. 



Length and breadth the same, 0'45 inch. 



This very elegant little shell is distinguished by its peculiar ribbing 

 from all its fellows ; the species which come nearest to it are O. fis- 

 sicosta and O. plicatella of Hall, and 0. ActonicB, Sil. Syst. 



Found in the upper division of the Lower Silurian formation at 

 Sazes and the Porto de Santa Anna, in the Serra de Bussaco. 



Plate VIII. fig. 2 a. Dorsal valve, exterior. 



Fig. 2 b. Dorsal valve interior. 



Fig. 2 c. Ventral valve, exterior. 



Fig. 2 d. Ventral valve, interior. 



Orthis Bussacensis, n. s. Pl. VIII. fig. 3. 



0. testa trausversim ovata, costata; costis numerosis di- vel tri-chotomis, 

 lateralibus confertissimis, mediis rarioribus. 



Shell transversely ovate, with both valves slightly convex and simi- 

 larly ornamented with numerous fine ribs, which subdivide two or 



