580 THE PALEONTOLOGY OF MINNESOTA. 
(Nuculide 
may be produced, without materially affecting the arrangement of the teeth, by the 
development of a small pit immediately beneath the beak (see plate XLII, fig. 80), 
or the teeth may be so arranged that the two series of teeth are directed at almost 
right angles to each other (see plate XLII, figs. 39, 90, 101 and 102). Finally, the 
shell is often very thin and the muscular scars barely distinguishable, while in other 
forms the shell may be thick and the muscular imprints exceedingly strong. 
In the present work I have allowed all these divergent types to remain under 
the single genus Ctenodonta. This does not, however, say that 1 could not have sub- 
divided the genus into several, nor that I do not believe that such a course will 
eventually be considered not only possible but desirable. Meek and Worthen long 
ago* expressed themselves as favoring a separation of the subtriangular forms like 
C. alta Hall, from the more typical ovate or elongate species. And Dr. 8. A. Miller} 
quite recently proposed the new genus Paleoconcha for one of the species of that 
group. He did so, however, under the misapprehension that the hinge of the species 
described by him is edentulous; so his evidence on the point is much weakened, for he 
would, most likely, not have proposed his genus had he understood the hinge fully. 
For the reason about to be mentioned, I am probably in a better position than 
any one else to speak of the possible subdivisions of this genus, namely, my efforts 
to collect a large mass of material have been successful, not only in the way of 
individuals, but in adding very greatly to the number of known species. Indeed, 
the Lower Silurian species in my cabinet outnumber the forms described previous 
to 1890 more than two to one. I believe, therefore, that with the careful study that 
has been given to this abundance of material, I am able to discriminate in a fairly 
trustworthy manner between the important and unimportant characters, to approx- 
imate truth in my views of the inter-relations of the species and to understand some 
of the genetic questions involved in the development of the family. 
The Lower Silurian species may be arranged in six more or less well marked 
groups, as follows: ; 
I. C. nasuta group. 
Elongate shells, narrow posteriorly, beaks subcentral; muscular scars moder- 
ately or distinctly impressed, hinge but slightly arcuate, teeth in a continuous 
series, straight or bent outwards. 
Species:. nasuta Hall, nasuta, var. robusta Ulrich, subnasuta Ulrich, oviformis Ulrich, 
cuneiformis Ulrich, regia, n. sp., ttennesseensis, n. sp., appressa, n. sp., crandalli, 
n. sp., iphigenia Billings. 
* Geol. Sur. Ill., vol. iii, p. 309; 1868. 
+ North American Geology and Paleontology, p. 498; 1891. 
+The new species, which are not described in this work nor in vol. vii of the Ohio Geological Survey reports, are 
marked simply as n.sp. Descriptions of these forms will, it is hoped, be published at an early date, the plate on which 
they are illustrated being ready for publication, 
