250 rUOCKEDlNGS OF THE NATIONAL MUSEUM. voi. xxviii. 



a broad rounded umbo, terminating- in a minute beak curving over the 

 area, to an erect, vertical, more or less rounded a[)ex, with a broad 

 base rising abruptly from the elevated umbo; a shallow, rounded, 

 median sinus occurs on most individuals, but it is sometimes absent. 

 Area high, and crossed by transverse lines of growth; it is usually 

 inclined over the hinge line, but it may be vertical or inclined for- 

 ward; it is divided by a strong delthyrium, which is covered ])y a con- 

 vex deltidium that the author of the species. Doctor Billings, describes 

 as perforate at the apex." 



I have not been able to verify this in a large collection of material from 

 various localities; casts of the interior of the extended beak indicate 

 but do not prove that there was a perforation; the front edge of the 

 deltidium is arched so as to leave a space of variable height between it 

 and the plane of the hinge line; in three examples the area is divided 

 into three parts 1)}^ longitudinal lines, two of the lines ])ound the del- 

 thyrium, and one on each side corresponds in position to the "' flexure "" 

 lines in (JhoJiiK and Ilippariony.r. A cast of the interior of a low ven- 

 tral valve shows a broad delthvrium, strong teeth, and supporting den- 

 tal plates, which are produced on the inside, so as to form a short, 

 elevated base (pseudospondylium), pi-obably for the adductor nuiscles, 

 and on the outside the plates are continued partially about the space 

 occupied by the points of attachment of the diductor muscles. 



The dorsal valve is moderately convex at the uml^o, sloping gently 

 from there to the margins; usually the slope is (convex, but in one 

 example it is slightly concave; area narrow and vertical or slightlv 

 inclined over the hinge line; casts of the interior show the crura, 

 points of attachment of posterior adductor scars, and area of attach- 

 ment of diductor scars. Vascular and ovarian markings unknown. 



Observations. — This shell has a wide geographic distribution. I have 

 collected it at the type locality in the township of Georgia, Vermont; 

 at Bic on the lower St. Lawrence River, and near the city of York at 

 Emigsville, in central Pennsylvania. The matrix at Bic is a finely 

 granular, slightly arenaceous limestone, in which the outer form of 

 the shell is well preserved, but the spines are rarely seen; at Swanton 

 and Georgia, Vermont, the shell occurs in siliceous limestone and are- 

 naceous shale, and a cast of the outer surface shows the spines; the 

 material from the tinely arenaceous limestone at the Emigsville locali- 

 ties, discovered by Mr. E. Wanner, of York, is the best preserved, 

 and affords excellent casts of the interior and exterior of the valves. 

 In all of the localities the variation in the form and elevation of the 



« Through the courtesy of Dr. J. F. Whiteaves, of the Geological Survey of Canada, 

 I had the opportunity of examining the types of Orthisina festinata. None of them 

 preserve the apex of the deltidium, so it is impossible to determine upon what Mr. 

 Billings based his statement that the deltidium is perforate. 



