EARLY DEVOXIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 1 89 



size, and distant from each other from one fourth to one half a line. No large striae or 

 ribs on the umbo. A second specimen has five or six large ribs, commencing at the beak 

 and dying out before reaching the most prominent point of the umbo. The remainder of 

 the surface covered with minute striae, and larger lines, somewhat like those of the last 

 specimen, but with concave, instead of flat spaces between them. 



The striking character of this species, shown well in Billings's figure 

 but insufficiently emphasized in the description, is the bunching of coarse 

 ribs at the beak and umbo. To start with, the shell presents a single broad 

 median rib with three or four of lesser size on each side. These are at first 

 separated by narrow furrows, but with growth the ribs broaden and become 

 less distinct and the furrows likewise widen. With the completion of 

 maturity these strong and few ribs may have become obsolete over the mar- 

 ginal portion of the shell, but in some specimens they are retained in force 

 to a much later growth stage than in others. Superimposed on these ribs 

 and becoming more conspicuous with the obsolescence of the former are 

 fine elevated radial lines with a group of four to six smaller lines between 

 each pair of the primary series. On the dorsal valve is a median furrow 

 with four ribs on each side and the same superimposed ornaments. 



On the interior of the ventral valve the cardinal denticulations are 

 developed on only about two thirds the length of the hinge line from the 

 deltidium. The muscle scars are separated by a strong and short septum. 



The interior of the dorsal valve shows a low bifurcate cardinal process, 

 hinge with fine denticulations not extending to the extremities ; the muscu- 

 lar area is small but well defined, concentrated posteriorly, with a prominent 

 short septum and lateral septa dividing obliquely the anterior and posterior 

 muscle scars. 



Mr Billings called attention to the relations of this shell to S t r o p h e o- 

 donta demissa, which in early stages has strong riblets or bunches of 

 striae in the umbonal region. Another species in which this character is 

 better expressed and longer retained and to which we find an approximat- 

 ing shell in the Grande Greve limestone, is Strop h. parva Hall, from 

 the Schoharie grit of New York. Further reference to the character of 

 this species is made above. 



Dimensions. The specimen on which Billings based his figure and 

 measurements was much larger than any we have observed. To this he 

 ascribes a length of 14 and a width of 13 lines. Our average specimens 

 have a length of 14 mm and a hinge line 16 mm in length. The largest we 

 have observed has a length of 18 mm and a width of 22 mm. 



Localities. The original specimens were from Indian Cove, in lime- 

 stone no. 8. Our examples are from the same localit}' and thence westward 

 along the coast exposures to Fruing's. 



From the typical aspect of these shells we observe departures in two 

 directions, the first and simpler due to breaking up of the original sparse 



