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



RiBEIRIA PHOLADIFORMIS, n, S. Pl. IX. fig. 17. 



R. testa compressa, subovata, autice rotundata, postice attenuata; lineis 



concenti-icis crebris inaequalibus. 



Shell subovate, laterally flattened, rounded at the anterior extre- 

 mity, and somewhat narrowed posteriorly ; opening narrow, extending 

 round three sides of the shell ; surface covered with sharp unequal 

 concentric lines. 



Length 1 inch ; depth ^ an inch ; thickness ^ of an inch ; width 

 of opening -^ of an inch. 



From the lower division of the Lower Silurian formation of the 

 Serra de Mucela and the Serra de Bussaco. 



Plate IX. fig. 17 a. Exterior. 



Fig. 17 b. Internal cast, side view. 



Fig. 17 c. Internal cast, dorsal view. 



Theca Beirensis, n. s, Pl. IX. fig. 19. 



T. testa depressa, vaginiformi, lineis transversis injiequalibus ornata, antice 

 subrotundata, postice angulato-sulcata, angulis lateralibus rotundatis. 



Sheath straight, tapering to a blunt point ; marked at irregular 

 intervals with sharp transverse lines of growth, with an angular 

 furrow down the middle of the back ; rounded in front ; lateral 

 angles rounded ofi'. 



Length 1^ inch ; greatest breadth ^ inch. 



Abundant in the Lower Silurian rocks of Riba de Baixo, in the 

 Serra de Bussaco. 



Plate IX. fig. 19 a. Front view. Fig. 19 6. Back view. 



Fig. 19 c. Cast of interior. 



DiTHYROCARIS ? LONGICAUDA, U. S. Pl. VII. fig. 3. 

 D. Cauda elongata, trispinosa, spina media longissima. 



Only the tail of this specimen is preserved, and it is referred to 

 Dithyrocaris doubtfully ; it is formed of two parts, the upper simple 

 and rounded, the lower formed of three lancet-shaped pieces, of 

 which the middle one is somewhat rounded and twice as long as the 

 lateral plates, which are nearly flat. In the species hitherto figured 

 these three caudal appendages are nearly equal in length. 



From the upper division of the Lower Silurian formation at Sazes, 

 in the Serra de Bussaco. 



Appendix C. — Notes on the Trilobites. 

 By J. W. Salter, Esq., F.G.S. 



In the list of Trilobites at p. 141, several species will be recognized 

 as identical with those from the Lower Silurian rocks of France ; 

 and in the case of Illcenus giganteus, I have ventured to reunite a 

 species found both in Portugal and Spain with the common form 

 which attains such large size in the slates of Angers. 



1. Illceims giganteus, Burm. (/. Lusitanicus, Sharpe, Geol. Journ. 

 vol. V. p. 146) . In the proportion of the axis to the sides of the body, 

 and in the position of the fulcrum, as well as the converging axal 

 furrows in the head and tail, the species seem to agree closely ; and 



