DIELASMELLA 281 



region similar to that of the opposite valve; the beak pointed, incurved 

 beneath the base of the pseudodeltidium of the opposite valve. Internally 

 the characters are in accord with the description of the generic characters 

 already given. 



Surface of both valves nearly smooth, marked only by fine, obscure, 

 concentric lines of growth. Shell structure finely punctate. 



Remarks. This little species was originally described as a member of 

 the genus Eunella. It agrees with Eunella in having a short brachidium, 

 but it has not been possible to determine whether the loop is of the 

 Eunella type, viz: with subparallel descending lamella?, or whether it re- 

 sembles the Dielasma loop in which the descending lamellae are divergent. 

 Eunella, however, is defined as having a distinct hinge-plate similar to that 

 of Cryptonella or Crancena, while in this species no true hinge-plate is 

 present, as has been pointed out in the definition of the genus, the trans- 

 verse connection between the bases of the crura being reduced to a narrow 

 band. 



Horizon. Glen Park limestone of the Kinderhook. 



DIELASMELLA CALHOUNENSIS n. sp. 

 Plate XXXI, Figs. 1-4 



Description. Shell small, compressed, lenticular, subovate to ovate- 

 subpentagonal in outline, longer than wide or sometimes about as long 

 as wide, the greatest width near or a little anterior to the middle. The 

 dimensions of a nearly complete specimen are : length of pedicle valve 

 10.5 mm., length of brachial valve 9.5 mm., greatest width 8.5 mm., thick- 

 ness 4 mm. 



Pedicle valve depressed convex, the greatest convexity near the umbo, 

 the surface nearly flat or gently convex towards the lateral margins, 

 abruptly inflected along the postero-lateral margins to the cardinal ex- 

 tremities; the mesial portion of the valve slightly flattened along an in- 

 definite, rather narrow band from in front of the umbonal region to the 

 anterior margin and frequently depressed along the median line of this 

 band in a shallow but distinct median sinus; beak pointed, nearly erect, 

 pierced by a subcircular foramen which encroaches largely upon the um- 

 bonal portion of the valve, only the margin being included in the delthy- 

 rium ; the delthyrium entirely closed by deltidial plates except at its apex, 



PIG. 35. A series of seven cross-sections of the rostral portion of the brachial 

 valve of Dlelasmella calhounensis (X 2%), showing the development of the 

 crural lamellae from the inner walls of the hinge sockets, and the narrow band 

 joining the bases of the crura just before they become free. 



