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



PORAMBONITES RiBEIRO, n. S. Pl. VIII. fig, 7. 



P. testa transversim elliptica, subventiicosa, punctata, Isevi; umbone dor- 

 sal! subproducto ; punctis impressis lineas radiantes concentricasque 

 describentibus. 



Shell transversely elliptical, with the dorsal valve somewhat pro- 

 duced at the beak, giving that valve rather a pentagonal outline ; 

 both valves moderately and equally convex ; smooth ; surface every- 

 where impressed with small punctures visible to the naked eye, which 

 are arranged so as to form lines radiatmg from the umbo and con- 

 centric rings. Interior of each valve furnished with two strong 

 dental plates, the bases of which are near together and parallel, and 

 which extend more than half-way along the length of each valve. 



Length 1^ inch ; breadth 1| of an inch. 



Found in the upper division of the Lower Silurian rocks at the 

 Porto de Santa Anna, m the Serra de Bussaco. 



The collection sent by M. Ribeiro, after whom the species is 

 named, contains several specimens with impressions of the interior 

 and exterior of each valve of this interesting shell : the matrix is so 

 friable that all are imperfect, but enough remains to show its prin- 

 cipal characters. It differs from all the Russian species in having 

 no indication of either sinus or mesial fold, in which it agrees with 

 the species next to be described, 



Plate VIII. fig. 7 a. Exterior. Fig. 7 b. Internal cast of dorsal 

 valve. 



Fig. 7 c. Internal cast of ventral valve. 



Fig, 7 d. Portion of surface magnified. 



Porambonites lima, n. s, Pl, VIII, fig. 6, 

 P. testa transversim ovata, subventricosa, tubulis minutis exsertis aspera. 



Shell transversely elliptical, convex ; surface rough with minute, 

 close-set, projecting tubes ; dental plates large. 



Breadth 1| inch. 



Found in the upper division of the Lower Silurian formation at 

 the Porto de Santa Anna, in the Serra de Bussaco, 



Only one imperfect specimen has been seen, so crushed as not to 

 show the outline complete : the form was probably nearly that of the 

 last species, from which it differs in having the punctures produced 

 into small slightly projecting tubes ; the dental plates are similar to 

 those of the last species, 



Plate VIII. fig. 6 a & b. Internal cast and exterior of one spe- 

 cimen. Fig. 6 c. Portion of surface magnified, 



Lept^na Beirensis, n. s. Pl. VIII. fig. 8. 



L. testa tnincato-elliptica, convexiuscula, costata, concentrice lineata; 



costis numerosissimis tenuibus, insequalibus. 



Shell truncato-elliptical, very slightly convex, covered with very 

 numerous narrow radiating ribs separated by rounded furrows ; the 

 ribs increase in number by the frequent insertion of thinner ribs 

 between the others, and are crossed by numerous closely set concen- 

 tric lines, besides which there are two strongly-marked lines of growth 



