LAMKI.U.msN, HI.V. S 



posterior edge ohli,,ii,.ly truncate, very p-ntly convex above the narrowly rounded 

 oasal partand passing n.tlMM-al.n.i.tlyii.t, >!., hinge line. Beaks not promim-nt. com- 

 pressed, >itu;it<-i between ooe-fifth and ooe-dxth of the length of the ihefl behind tiu> 

 anterior extremity. Surface of cast showing moderately di.stim-t irregular concen- 

 tnr undiilatmn*. u Inch are most strongly defined on the umhonal ridges and on the 

 il:itf>iu..i oreonemTfl Hank-. These are crossed on the cardinal slopes by two obscure 

 Miln. Anterior iniiM-tilar scar moderate in size and definition, ovate; small pedal 

 musriilar scats ili>tinct above them. 



The val\.>> of this shell gape very slightly posteriorly and pn.l.altly also in front. 

 l)iit upon tliis point the material at hand presents no conclusive evidence. They 

 have also the point of greatest convexity a little behind the center. Both of these 

 features are characteristic of Orthodesma. On the other hand the species presents 

 conM.lt>ral.le resemblance to the early elongate forms of Mn,li,,lopsis like M. arguta, 

 1'iit this indicates, I believe, merely, what I have already stated, a common origin 

 for the two genera and not that 0. subnasulum was evolved from the Modiolopsis. I 

 come to this conclusion because the present species is even nearer the 0. ntinnesottiue 

 which occurs in the same beds holding M. anjiiln. Further, as regards the develop- 

 mental history of Orthodcsma, I view 0. minnesotense and 0. subnasulum as the earliest 

 known stages in the line of development that produced 0. curvutum II. and W., and 

 one or two undescribed species occurring at Cincinnati, Ohio, while 0. rectum H. and 

 \V., appears to have been derived through intermediate species from 0. sdiuchfrti. 



Specifically 0. subnasutum is distinguished from O. minneaotense by its more 

 elongate and posteriorly diverging form, better defined mesial depression, the longi- 

 tu.linal sulci on the cardinal slope, and more distinct concentric folds. 



As regards the name of the species, it will be seen from the synonomy that 

 Meek and Worthen first called it Modiolopsis subnasuta, being evidently unaware 

 that the same name had been used previously by Prof. James Hall (Can. Nat and 

 Geol.. vol. v, p. 148; 1860) for an Upper Silurian species from Canada. This fact 

 '"ing brought to the notice of Prof. Worthen he, in 1882, proposed to change the 

 name to Mmlwhpxis carrollensis, and this specific designation will have to be used 

 -lioulcl the Canadian species also prove to be an Orthotleanm. Hut until that has been 

 established, the original name will have a clear field. 



Cation and loeality. Thf tvp.-s i,r the specie* are from the Galonm of Carroll county, IlllnoU. 

 -prclniun here figured and deocribed Is from the same horizon near Dlion, IlllnoU. lo Minnesota 

 the specie* to to be looked for In the "Madura beds" < -na. 



