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PALAEONTOLOGY OF NEW YORK. 
verge, making a very abrupt slope on their outer margins, but a gentle slope 
inward. The median septum is very faint until it meets the lateral ridges, 
when it widens over the anterior portion of the shell. Between each ridge and 
the median septum the surface is marked by radiating muscular lines which 
indicate the position of the anterior adductors, and at the posterior extremity 
of each ridge lies a circular impression, undoubtedly of muscular origin, but 
whose function is not disclosed by comparison with allied forms. Our present 
knowledge is insufficient to determine the degree of development of these lateral 
muscular ridges throughout the “ Discinas,” but in the typical forms upon 
which we have based the discussion of the foraininal characters they do not 
appear at all, and we are disposed to regard them at least as of subsidiary generic 
importance, and propose to place this Hamilton species under the sub-generic 
term 
Lindstrcemella, sub-gen. nov. (See Plate IY e, figs. 25-28.) 
Diagnosis. Shells with outline, contour and pedicle-characters as in Orbi- 
culoidea. Brachial valve with a faint median septum and two strong approx¬ 
imating ridges or muscular fulcra, beginning just behind a transverse line 
passing through the apex and rapidly converging to meet the median septum. 
Anterior adductor scars lying between these ridges and the median septum; 
a circular muscular scar at the posterior extremity of each ridge. 
In the pedicle-valve, the foramen has essentially the structure of Orbiculoi- 
dea, but its distal margins, though lying in close juxtaposition appear not to 
have been united. 
Type, Lindsimmella aspidiwn, sp. nov. # 
In the pedicle-valves of “ Discinas ” are often seen traces of what, at first 
sight, appear to be similar, though faint ridges, situated, however, further back¬ 
ward, meeting at the apex and diverging along the pedicle-groove. We are 
satisfied that this feature is produced by the flattening of the shell about the 
more or less thickened internal pedicle-area and is purely accidental. In several 
of M. Barrande’s figures this feature is made to appear in various degrees of 
For description see Supplement. 
