-.,, . LAMELLlBEANCHIATA. 5l9 



orthodesma subnasutum.] 



posterior edge obliquely truncate, very gently convex above the narrowly rounded 

 basal part and passing rather abruptly into the hinge line. Beaks not prominent, com- 

 pressed, situated between one-fifth and one-sixth of the length of the shell behind the 

 anterior extremity. Surface of cast showing moderately distinct irregular concen- 

 tric undulations, which are most strongly defined on the umbonal ridges and on the 

 flattened or concave flanks. These are crossed on the cardinal slopes by two obscure 

 sulci. Anterior muscular scar moderate in size and definition, ovate; small pedal 

 muscular scars distinct above them. 



The valves of this shell gape very slightly posteriorly and probably also in front, 

 but upon this 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 

 considerable resemblance to the early elongate forms of Modiolopsis like M. arguta, 

 but this indicates, I believe, merely, what I have already stated, a common origin 

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

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

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

 mental history of Orthodesma, I view 0. minnesotense and 0. suhnasutum as the earliest 

 known stages in the line of development that produced 0. curvatum H. and W., and 

 one or two un described species Occurring at Cincinnati, Ohio, while 0. rectum H. and 

 W., appears to have been derived through intel:mediate species from 0. schucherti. 



Speciflca,lly 0. suhnasutum is distinguished from 0. minnesotense by its more 

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

 tudinal sulci oil 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 

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

 being brought to the notice of Prof. Worthen he,. in 1882, proposed to change the 

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

 should the Canadian species also prove to be an Orthodesma. But until that has been 

 established, the original name will have a "clear field. 



Formation hnd locality. — The types of the species are from the Galena of Carroll county, Illinois. 

 The specimen here figured and described is from the same horizon near Dixon, Illinois. In Minnesota 

 the species is to be looked for in the "Maclurea beds "of the Galena. 



