1853.] RIBEIRO AND SHARPE — BUSSACO. 157 



at a considerable distance apart. Hinge-line equal to the greatest 

 width of the shell. 



Length I'l, breadth 1*2 inch. 



Found in the upper division of the Lower Silurian formation at 

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



This shell closely resembles the next described, L. ignava, and 

 perhaps they may prove the opposite valves of one species. 



Plate VIIL fig. 8 a. Dorsal valve, exterior. 



Fig. 8 b. Dorsal valve, internal cast. 



Lept^na ignava, n. s. Pl. VIII. fig. 9. 



L. testa truncato-elliptica, planiuscula, costata, concentrice lineata et cor- 



rugata ; costis dichotomis, ad apicem rotundatis, deinde subobsoletis. 



Shell truncato- elliptical, very thin and nearly flat, with the hinge- 

 line almost of the greatest width of the shell, ornamented near the 

 umbo with numerous rounded bifurcating ribs crossed by indented 

 lines ; farther from the umbo the ribs become fainter and gradually 

 obsolete, while the concentric lines become irregular waving wrinkles. 



Length 1 inch, breadth the same. 



Found in the upper division of the Lower Silurian beds at the Porto 

 de Santa Anna, in the Serra de Bussaco. 



Only one valve of this shell has been seen, and it may perhaps 

 prove to be the ventral valve of the preceding species, L. Beirensis, 

 to which it is very nearly allied. 



Pleurotomaria Bussacensis, n. s. Pl. IX. fig. 18. 



P. testa Iseviuscula, ambulacris sex, junioribus depresso-rotundatis, ultimo 



subangulari ; apertura sinu medio profundiusculo. 



Shell smooth, with six rounded whorls separated by a deep suture ; 

 the upper whorls slightly depressed, the last somewhat angular in the 

 middle ; opening with a deep sinus corresponding to the angle on 

 the whorl. 



Height 1 inch ; greatest diameter 1 inch. 



Found in both divisions of the Lower Silurian formation of the 

 Serra de Bussaco. 



RiBEiRiA, Sharpe. 

 Testa univalvis, elongata, lateraliter compressa; apeiiura elongata, an- 



gusta : intus lamina transversali anteriore et impressione musculari ele- 



vata elongataque munita. 



Shell univalve, elongated, laterally compressed into the form of a 

 Pholas or Lithodomus ; open at both ends and along the pedal margin, 

 with athick transverse internal plate nearthe anterior extremity, behind 

 which is a very large corrugated boss for the attachment of a muscle. 



This curious shell appears related to the family of Calyptrceidce, 

 but it shows no trace of a spiral growth ; as far as can be judged 

 from the imperfect specimens seen, it is equilateral, and both the 

 transverse internal plate and the muscular attachment are placed 

 along the middle of the back of the shell ; the external form may 

 be described as a Calyptrsea pressed together laterally till the sides 

 nearly meet, leaving only a narrow opening for the foot of the animal. 



The genus is named after Senhor Carlos Ribeiro. 



M 2 



