::intf tli> -lit. perhaps not OD6 specimen in ;i thon-aiid of the li. sulcntina 

 section preserve- it entirely. As a rule the delicate apertural portion of the shell i- 

 i.niki-n .iu.iv quite to the posterior end of tln> -lit. In tlie other section a complete 

 aperture i- a much more frequent occurrence, l>oth because the slit is shorter and 

 the hf 11 -tronger. 



It will l>e noticed that the characters brought out in the foregoing paragraphs 

 are different from those pertaining to much the greater part of the genus as defined 



A'aagen and Koken. They derived their ideas of the genus chiefly from Upper 

 Silurian, Devonian and Carboniferous spirally ribbed species, which with few 

 exception- (none of them true liucairia) will fall into the genus that we propose to 

 call , M.S. It is to be noted, however, that Koken with his usual acumen 



draws attention (op. ',(., p. 380) to differences in the aperture and surface sculpture 

 between the " ^nlr,itinn-\\\<\\^" and the Devonian and Carboniferous species. 

 I iifnrtiiii.it. 'ly ho did not, or for want of material could not, carry his comparisons 

 to their logical conrln-ion. 



According to our opinion I>n<;inia, as here restricted, is (1) strictly a Silurian 

 genus and possibly not even represented in the Upper Silurian, (2) it is the stock 

 from which .^nlpingostoma. and later Tremanotus, was derived, and (8) it is not 

 genetically related to AV- //< /'.. In support of the first statement we have the 

 that while the twenty known Lower Silurian species fit closely together, not 

 one of the succeeding forms could be included without materially altering the 

 generic diagnosis. The truth of the second statement is but too apparent to those 

 who are obliged to discriminate between imperfect specimens of associated species 

 { S'llfrinnnstoma ami />'i. //,/. Casts of the former from which the abruptly 

 expanded aperture has been broken away, and on which the dorsal fissure is not 

 clearly represented, are most difficult, if indeed it is at all possible, to separate from 

 casts of liii>-<iia. The important agreement, however, lies in the surface sculpture 

 which in all essential respects is the same in the two genera. Hut it is scarcely 

 necessary to di.ncu tin- relation of lincania to .W//i//<>.vtoi here since we shall do 

 so quite fully some pages hence in our remark- on that p-nns. The third statement 

 refers t< Tlii- ^enn- was evolved, we think, not from Hn"inia but from 



Ibllfrophon. No better description of the genus could be given than that which says 

 that it includes species agreeing in all respects with l:Jl.;-ujihon excepting that t Ill-s- 

 hare revolving stri.r which, with tin- transverse lines, produce a cancellated sculpture. 

 Without the revolving lines Huranopsis would be nothing more or less than /.V//<'/M/I//O;/. 

 Not so, however, with /. -ince that genus would still IH> distinguishable. 



Again the spiral lines are not the same in numni-i and hucanopsis, bein^ -tr.uj/lit 

 in the latter and not oblique nor wrinkled nor ever interrupted as in the former. 



