184 PALEONTOLOGY OF NEW YORK. 



verge, making a very abrupt slope on their outer margins, but a gentle slope 

 inward. The median septum is very f;iint 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 foraminal 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 IV e, figs. 25-28.) 



Diagnosis. Sliells 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, Lindstramella aspidium, 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. 



