LAMELLIBRANCHIATA. 519 
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 O. subnasutwm was evolved from the Modiolopsis. 1 
come to this conclusion because the present species is even nearer the O. minnesotense 
which occurs in the same beds holding M. arguta. Further, as regards the develop- 
mental history of Orthodesma, I view O. minnesotense and O. subnasutum as the earliest 
known stages in the line of development that produced O. curvatum H. and W., and 
one or two undescribed species occurring at Cincinnati, Ohio, while O. rectum H. and 
W., appears to have been derived through intermediate species from O. schuchertv. 
Specifically O. subnasutum is distinguished from O. minnesotense by its more 
elongate and posteriorly diverging form, better defined mesial depression, the longi- 
tudinal 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 
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 and 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. 
