66 THE CHICAGO ACADEMY OF SCIENCES. 
**Shell small, rays narrow and very numer- 
OUS Soe acces igidscie > alae ee ena multiradiatus 
7tShell compressed, rays wide with namerous 
finer-ones, between. . .... .c eae ee eenee ligamentinus 
c. Shell not alate, rays made up of zigzag dashes. 
+Shell elliptical, posterior end very pointed 
atid with a‘slight shoulder. 325-3. sacra donaciformis 
++Shell trigonal, posterior end not so pointed 
as +, and with a decided shoulder and also 
a heavy, elevated ridge, extending from the 
umbones to the ventral margin................ elegans 
B. Shell smooth without rays. 
a. Shell large, long and narrow, laterally compressed......... gibbosus 
b. Shell large, more or less quadrate, short and high, 
umbones depressed. 
1. Shell almost quadrate, generally compressed, 
hinge teeth rather light, and cardinal teeth 
widely ‘diverping.ccs cccec cease ce elec sine sees « rubiginosus 
2. Shellroundly elliptical, produced at the posterior 
end, hinge teeth solid and heavy, and cardinal 
teeth placed near together...3........000ces0- coccineus 
c. Shell large, trigonal, umbones very much elevated and 
WA Ated so 2. wc nie aw alo Be ee aes ae ee ea trigonus 
d... Shell. very small; elliptical, thiny 5 i351, 35s cepee tee neo parvus 
C. Shell large, surface coarsely wrinkled. 
a. Umbones much elevated and inflated, posterior end 
truncated; shell inflated. 1.5 djce cece 1 eee cee eae plicatus 
b. OUOmbones depressed, not inflated, posterior end triangu- 
lar,‘shell.compresseds.22.o2-00c0~ so dav> ne cue Hele Swe undulatus 
D. Shell pustulose. 
a. Pustules few and in the form of nodules........... ......: reflexus 
b. Pustules numerous. 
1. Shell compressed, orbicular, ventral border 
rounded, pustules scattered over the surface. .verrucosus 
2. Shell compressed, quadrate, ventral border ex- 
cavated, pustules arranged in two series on 
either side of a depressed bare spot.......... lacrymosus 
3. Shell inflated, orbicular, pustules scattered over 
the shell with no regard to system; pustules 
DICSEHMOT ADSENES. wham tes eee: iene pustulosus 
Genus STROPHITUS Rafinesque, 1820. 
Shell: Inflated, rounded, rhomboid, rather thin, with a hard, 
shining, often rayed epidermis; beak sculpture consisting of a 
few coarse subparallel ridges, which bend slightly upward pos- 
teriorly and are heaviest at the posterior angle of the shell with 
fine radiating lines in front of and behind these ridges; hinge 
line bent inward in front of the beaks; teeth rudimentary, 
usually consisting of an irregular compressed ridge, which is 
