M..I I. \MKI.I. II li\N, III \ 



M. HI.,.//,,/,;, , t ,| x| 



fe ben iMtriotod a...i defined, thii ,. .,, 1 ,t l , llte8a we ,,. mirko( , , , 



low,-,- 



,!,. -hell, Th- old, .... klp ^ 



I'lack Kner dirttom Of kUTVeoto. fnrmati,,,, Some of these an- , v .| 



th.t. i,v iradnaJ m Mi.ii.MtjnnHof the base, evolved specie* of the tf. , 



Utt, '.mo there existed elon^u- forms like i/ 



m.-ommon v.tho,,/;,,,/,,,,,,, t hat we cannot do,,l,t that they indicate th, primil 

 -t,,ck from whirl, .i/.../,../.. r ;. ;lll ,| orthodetma were evolved. The ./. .,,/ lllie 

 ntim,,.d and formed a reasonably complete chain through M 



M. >,,,>//.,. aii.l one or two undescribed tpecies of the middle lwd of the 



'mnnnati K f (11I| , illtll .,/. , ,,, . ;U1(| w & common ^^.^ ^ ^ ^^ ^ 



..f that seru-s ,,f r M-k, in Ohio and Indiana, and in- Aliens from equivalent 



trata in Minnesota. I., this case the form was shortened, the anterior end particu- 

 larly. In tlu> .17. m ,,,lini lir i* line, however, the changes were different Here we may 

 o with M. >/Hi,7,Xanoval form with the posterior end broadly round.-.] an.) widest 

 Tin- MM-ins to have Rone over into an uppi>r Trenton species (M.subreda Ulricb, Ms.) 

 having a murh narrower posterior end-indeed, the back and base are nearly pamllt-l. 



M.-xt follow the type hy easy stages through varieties occnrrinK in the I 

 horizon to the normal form of M. modiolaris. Much indeed might be siid upon these 

 not only interesting hut important questions of evolution, and nothing would please 

 me more than to be allowed to demonstrate the positions here outlined. Hut tim,- 

 and space are lacking, and the few points made are offered chiefly in the ho|* that 

 the suggestions may stimulate students to researches in similar lines. The Held i 

 inviting and the results to be obtained all important. 



The relations of the genus to the other genera of the family treated of in tin- 

 chapter will be discussed in the remarks following their descriptions. 



No compari-ion of .J/.,,//o/o//.--ix and Modiomorpha, Hall, has, so far as I can learn. 

 ever been publi>hed. This is strange, since the 8|>ecies of the two genera are strik- 

 ingly similar. As a rule it seems they are regarded as differing widely, but in what 

 n-pecte we are not informed. Mr. S. A. Miller, for instance, places them into two 

 di-tinct families, but fails to state his groun.N for the separation.* A mistaken idea 

 seems to prevail where it originated I cannot say that the hinge of Modioloptu 

 has lateral teeth, and this i a- the principal difference I >et ween the two genera 



by Nettleroth.; 



Now. let ii- tee what differences really exist between tln-m Taking Modiomorpka 

 ''"" i ' -eut.ttive of the Devonian genus, we find that, to Car as external 



characters are concerned, it would pass very well for a species of ;en 



. .-iii-l ralaH>lolur. |l IS*; M*. 



