442 NEW YORK STATE MUSEUM 



lations (which are concentric, prominent narrow ridges separated 

 by concave transverse interspaces, twice the width of the ridges)^ 

 their closeness of arrangement (8 in 20 mm where the width of the 

 conch is 8 mm ; 7 where it is 10 mm and 6 in the same space where 

 it is 13 mm), they fully agree with the figure and description of 

 O . 1 a m a r c k i as furnished by Billings. A conclusive identifica- 

 tion of these specimens whose septa and siphuncle are unknown is- 

 however impossible. 



Better m.aterial of a form which is identical in external characters, 

 has been secured in the uppermost beds of Beekmantown age ex- 

 posed at the Valcour shore south of Plattsburg. These beds are 

 equivalent to the Fort Cassin beds of V'^ermont. The specimens 

 obtained there [sec pi. 15, fig. 2-5] retain the septa as well as the 

 siphuncle and have furnished a basis for the following description 

 of the species. 



Description. Conch of but moderate size (greatest length ob- 

 served 120 mm, greatest width observed 25 mm), very slightly 

 ^^^ curved (the bight of the arch formed by a frag- 



r"'" "^ ment 58 mm long is but 2 mm), the curvature 



apparently somewhat increasing toward the ma- 

 ture part of the conch; very gradually expanding 

 ' ' (within 50 mm from a diameter of 8 mm to one 



of II mm, or not quite i mm in 20 mm) ; cross- 

 I i rC 5-ection subcircular. Surface in the apical part 



i^rovided with sharp longitudinal lines which in 

 more advanced stages of the conch are replaced 

 Fiff I Protoc cio ^^ annulatious. These increase in strength to 

 miiint^s (ip^)"^ Lon<^i^ ^^^^ cphebic conch where they are prominent, 

 urai"stze^'^'^'°"' ^^'' narrowly rounded ridges which pass straight 

 transversely around the shell. The width and 

 relative distance of the annulations increase slightly toward 

 the aperture (the latter more than the former) ; the average 

 width is about i mm and that of the concave interspaces a little 

 more ; there being 9 in the space of 20 mm where the diameter is 

 8 mm and but 4 within the same space in the widest specimen ob- 

 served. The surface ornamentation of the adult conchs has not been 

 distinctly discerned but seems to have consisted, on the interspaces 

 at least, of transverse lines. 



Siphuncle with fusiform segments; large (averaging one third 

 the width of the conch), propiocentren, more excentric in the 

 ephebic conch than in the preceding stages, being finally distinctly 



bh: 



