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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 . lamarcki as furnished by Billings. A conclusive identifica- 

 tion of these specimens whose septa and siphuncle are unknown is 

 however impossible. 



Better material 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 Vermont. The specimens 

 obtained there [see 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 



relative distance of the annulations increase slightly toward 

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

 width is about 1 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 



Fig. 15 Protocyclo- 

 ceras lamarcki 

 Billings (sp.) Longi- 

 tudinal section. Nat- 

 ural size 



curved (the hight of the arch formed by a frag- 

 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 11 mm, or not quite 1 mm in 20 mm) ; cross- 

 section subcircular. Surface in the apical part 

 provided with sharp longitudinal lines which in 

 more advanced stages of the conch are replaced 

 by annulations. These increase in strength to 

 the ephebic conch where they are prominent, 

 narrowly rounded ridges which pass straight 

 transversely around the shell. The width and 



