79 



I am not acquainted with the habitats of either of these two last men- 

 tioned fossils; but imagine, from the appearance of the matrix, that 

 they are from the neighbourhood of Dudley, in Worcestershire. 



XLI. Cydostoma. A discoidal or convexo-conical, nearly transparent 

 univalve, with cylindraceous turns. The opening round, with a cir- 

 cularly continued margin, suddenly and widely thrown back. 



The cydostoma, in their adult state, have their margins always re- 

 flected, which is not the case with the delphinula ; but they are more 

 certainly distinguished from each other by the thinness of the shell of 

 the cydostoma, and by their never having any nacre, which is always 

 possessed by the delphinula. Cydostoma spiruloides, Lam. is a minute 

 species of this genus, the form of which may easily be comprehended 

 without a figure ; since it is formed by three turns, spirally disposed, in 

 the same manner as in the planorbes : the last turn being so 'detached 

 from the rest, as to give it the appearance of a young nautilus spirilla. 



It must be observed, that there are some shells, not yet classed, whose 

 openings are round, and whose margins are united like the cydostoma; 

 but in which, unlike the cydostoma, the margins are not dilated nor 

 reflected, and which are of an elongated or turriculated form. 



Lamarck describes six fossil species : C. cornu pastoris, C. spiruloides, 

 C. planorbula, C. macrostoma, C. mumia, C. Turritdlata. 



XLI I. Scalaria. A turreted univalve, with acute longitudinal raised 

 ribs. The opening nearly circular; the margins uninterrupted, bor- 

 dered, and reflected. 



The raised ribs sufficiently distinguish the shells of this genus from 

 those of delphinula and cydostoma. 



I am not in, possession of any fossil of this or of the two pre- 

 ceding genera; nor do 1 know of any having been found among our 

 English fossils, Lamarck has discovered five species of this genus 

 among the fossils of Grignon and Pernes : S. crispa, S. decussata, S. denu- 

 data, S. monocyda, and S. plicata. 



XLIII. Twritella. A turreted univalve shell; the opening entire^ 



