82 NEW YORK STATE MUSEUM 



primitive feature appearing in the early growth stages (cf. K i o n o c. d a r- 

 w i n i) with their greatest relative strength. With continued shell growth 

 they become broader, less distinct and in progressed shells entirely obsolete 

 even before adult growth is attained. The longitudinal lines, however, 

 which develop before the early annuli, remain with the obsolescence of the 

 latter, for a time the principal feature of the exterior. Contemporaneous 

 with both of these surface features there develops a concentric lineation, 

 which also may become more conspicuous with the suppression of the 

 annuli, in some species becoming a prominent cancelating feature, even 

 entirely replacing the longitudinal ribs. Among species which have been 

 described from the American Upper Siluric the following express vari- 

 ous conditions of this combination of surface characters : O. u n d u - 

 latum Hall, O. annulatum Sowerby var. americanum Foord, 

 O. medullare Hall, O. nodocostatum McChesney, O. cadmus 

 Billings, O. virgatum Sowerby, O. 1 a p h a m i McChesney. This state- 

 ment does not impugn the differential specific values of certain of these 

 forms, but, as all of them have been described from incomplete cones, it is 

 clearly impossible with present knowledge to decide on the specific values 

 involved. Generic characters among these late Siluric species are conse- 

 quently highly obscured and uncertain, and the genera of this group pro- 

 posed by the late Professor Hyatt pass into each other, as do the specific 

 characters. Hyatt has proposed to term species which retain the annuli 

 throughout growth with concentric undulating lines or frills, Dawsonoceras 

 (D. annulatum Sow. O. undulatum Hall). Shells having the 

 longitudinal ridges conspicuous and obscure annuli in senile growth are 

 termed Kionoceras ; a condition in which the annuli of early growth become 

 later obscured, with corresponding increase in the prominence of the longi- 

 tudinal lines, are Spyroceras, while a development of spines or nodes at the 

 intersection of annuli and longitudinal ridges constitutes the character of 

 Thoracoceras. 



We find among the material from Rochester several small specimens 

 of sharply annulate cones which we refer to Dawsonoc. annulatum 



