CONTRIBUTIONS TO PALAEONTOLOGY. 



51 



with the thickened margin of the valve as in the other allied 

 genera. 



In the cast of the dorsal valve we have the mark of the median 

 septum, with an elongate lanceolate muscular impression, reaching 

 nearly to the middle of the valve. The imprint of the triangular 

 process, and the cavities made by the crura, are often preserved. 



The species of this genus may be readily distinguished from Merista 

 by the absence of the shoelifter process, which, in numerous specimens 

 compared, constitutes the principal difference between the two genera.* 



The following illustrations will serve to show more clearly the characte- 

 ristics of the genus : 



Fia. 27. 



Fig. 29. 



Fio. 27. Meristella nasuta = Atrypa nasuta (Conrad) = Athyris clara (Billikgs) 

 Dorsal view of a young individual. 



Fig. 28. An older individual. 



Fig. 29. Interior of the ventral valve. 



Fig. 30. 



Fig. 31. 



Fig. 32. 



Fig. 30. Cast of the ventral valve of M. nasuta. 



Fig. 31. Dorsal view of the same species. 



Fig. 32. Interior of the dorsal valve of M. arcuata, showing the hinge-plate and median 

 septum. 



• On Plates 39 & 41 of the Palgeontology of New-York, Vol. iii, may be found some 

 illustrations of tHe casts of species of this genus. 



