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PALEONTOLOGY OF NEW YORK . 
distance, like a narrow, triangular shelf, beneath which the foramen probably 
opened. The interior opening of the foramen is, however, not apparent on 
any of the specimens examined, for, as usually preserved, the matrix has 
adhered to this subapical cavity, and in a single example only, is the shelf-like 
character of the median area distinctly demonstrated. A faint longitudinal 
ridge passes from the apex of the shelf to the anterior margin, but no other 
markings are discernible on the interior except faint radiating or slightly un¬ 
dulating, probably vascular lines. 
The interior of the brachial valve, as far as known, shows no other charac¬ 
ters than the radiating lines, which appear to belong to the ornamentation of 
the external surface. 
Shell-substance tenuous, apparently corneous. External surface covered 
with more or less prominent, sometimes lamellose concentric growth-lines, 
crossed by fine, gently curved, radiating striae which are usually more prominent 
when the concentric lines are exfoliated. 
Type, Discinopsis Gulielmi, Matthew. 
The nearest alliances of this peculiar and imperfectly known fossil appear to be 
with the genus Linnarssonia ; both having the apical shelf or callosity in the pedi¬ 
cle-valve, but in the latter this is more elevated and concentrated posteriorly. 
In other respects, Discinopsis differs from this genus, as well as from Acro- 
thele, in the other internal markings and in the excentric apex of the brachial 
valve, while it is removed from Acrotreta by these features and also by the 
absence of a subapical area. The deep furrows in the pedicle-valve may be 
compared to the pair of muscular (?) furrows diverging from the foraminal 
opening in Acrotreta gemma , as shown by Walcott.* One of the figures given 
by this author under the designation, Linnarssonia Taconica,\ appears to 
represent a form of Discinopsis. 
* Palaeontology of the Eureka District, pi. i, fig. 1 b. 
t American Journal of Science, vol. xxxiv, pi. i, fig. 18 a. 1887. 
