106 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- 

 THEiiE, 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 sulaapical 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, Lmiarssonia Taconica,} appears to 

 represent a form of Discinopsis. 



* Palseontolog-y of the Eureka District, pi. i, fig. 1 b. 



t American Journal of Science, vol. xxxiv, pi. i, fig-. 18 a. 1887. 



