339 



8. JSfeiv Species of Fossils from the Caldferous Formation, with Re- 

 marks on some others previously described. 



INCERT^ SEDES. 

 Genus Ribeieia. CSharpe.) 



RiBEiEiA. (Sharpe.) Jour. Geo. Sac, vol. ix, p. 151, 1853. 



Remarlcs. — Mr. Sharpe thus describes this remarkable genus. " Shell 

 univalve, elongated, laterally, compressed into the form of a Pholas or 

 Lithodomus ; open at both ends and along the pedal margin, with a thick 

 transverse internal plate near the 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 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." (Sharpe, loc. cit.) 



One species, R. pholadiformis is described by Mr. Sharpe. It was 

 found in the lower division of the Lower Silurian formation of the Serra 

 de Mucela, and the Serra de Bussaco, in the neighborhood of Bussaco in 

 Portugal. 



•J. W. Salter has, I think, referred this genus to the Crustacea, but I 

 have not at present access to the publication containingliis paper. 



Our species agree, in all general characters with the description of Mr. 

 Sharpe, except that the internal casts do not exhibit the impression of any 

 large boss for the attachment of a muscle. This, however, may be due 

 to differences in the growth of the shell of the individuals examined. It 

 is not uncommon, in the Mollusca at all events, to find in the same species 

 individuals with the muscular scars strongly developed, and others with 

 it scarcely perceptible. In a fragment of a silicified specimen in our 

 collection, the transverse plate is perfectly preserved. Just beneath the 

 umbo, and in front of it, there is a small aperture, of a semicircular shape, 

 which appears to be the entrance to a tubular passage running backwards 

 over the transverse plate into the general cavity of the body. This is 

 visible in so many specimens that I think it scarcely possible that it can 

 be the result of the accidental destruction of the apex of the umbo or 

 beak. My present impression is that it served the function of a byssal 

 orifice, and that these species were anchored by a byssus passing through 

 the beak. 



