SCIENTIFIC INDEX. 
numitor, 738. 
perforator, 738. 
satyrus, 738. 
Nileus, 712, 713. 
affinis, 713. 
armadillo, 713. 
macrops, 713. 
minnesotensis, 712. 
palpebrosus, 713. 
scrutator, 713. 
vigilans, 712*, 713. 
Niobe, 704. 
Note on subgeneric classification of Ceraurus, 736. 
Nucleobranchiata, $19, 817. 
Nucula, 582, 478, 479, 486, 578, 582, 583. 
fabula, 607. 
fecunda, 595. 
lepida, 604. 
levata, 599. 
obliqua, 604, 
poststriata, 610. 
Nuculana, 479*, 486. 
Nuculida, 578, 486, 58], 582. 
Nuculites, 582, 486, 606, 610. 
Nudibranchiata, 818. 
Nyassa, 486. 
Observations on American Lower Silurian Phaco- 
pide, 732. 
Occipital groove, 697; ring, 697. 
Octonaria, 632. 
Odontocephalus, 733. 
Odontophore of Gastropoda, 814. 
Odontopleura, 744. 
parvula, 744%. 
Offa, 632. 
Omospira, 944, 931, 932, 933, 934, 938, 945, 1005, 
alexandra, 946. 
laticincta, 945, 932. 
Oncoceras, 798, 764, 766, 796, 799, 804. 
brevicameratum, 803. 
carveri, SO1, 812. 
constrictum, 802. 
douglassi, $O!, 812, 
exiguum, 798, 812. 
lycus, 799, 812. 
minnesotense, 798, 799*, 812. 
mumiaforme, 797. 
pandion, 802, 803, 812. 
Oncoceratide, 797. 
Onustide, 939. 
Oonoceras, 803. 
Operculum of Gastropoda, 816. 
Ophileta, 1030, 815, 934, 936, 937, 1023, 1025, 1026, 
1028-1030, 1031, 1033, 1038. 
bella, 1025. 
compacta, 769. 
complanata, 1025. 
ottawaensis, 937. 
owenand, 1032, 937. 
uniangulatus, 1025. 
Ophiletina, 1029, 1028, 1030, 1032. 
3 angularis, 1031, 1030. 
exlvii 
sublaxa, 1030, 1031. 
var. depressa, 1031. 
var. sequens, 1031. 
Ophisthobranchiata, 818, 817, 844, 845. 
Ophisthoptera, 628. 
Oreecardium, 486. 
Oriostoma, 1058. 
Ormoceras, 781, 794. 
backi, 781. 
bayfieldi, 781. 
remotiseptum, 782, 7883. 
tenuifilum, 781, 783. 
Orthis, 10387. 
flabellites, 798. 
pectinella, 736. 
perveta, 615. 
quadrata, 677. 
striatula, 769. 
subquadrata, 865. 
retrosa, 597. 
testudinaria, 768. 
Orthoceras, 783, 763*, 766, 776, 778. 
amplicameratum, 790, 783, 812. 
anellus, 784, 812. 
beloitense, 782. 
beltrami, 789, 812. 
bilineatum, 786, 765, 785, 787, 812. 
crotalum, 787. 
hastatum, 793. 
gregerium, 789. 
junceum, 790, 812. 
lesueuri, 785, 812. 
longissimum, 765. 
lycus, 799. 
matutina, 769. 
multicameratum, 789, 812. 
multitubulatum, 765. 
nicolleti, 784, 812. 
olorus, 788, 784, 785, 812. 
perroti, 785, 788, 812. 
planoconvexum, 791. 
servile, 793. 
sociale, 789, 812. 
Sp. und., 786. 
tenuistriatum, 788, 812. 
undulostriatum, 769. 
vertebrale, 788. 
ziphias, 798, 
Orthoceratide, 783. 
Orthodesma, 516, 477*, 478, 485, 502-504, 507, 514— 
519, 523, 525, 531, 618, 623. 
byrnesi, 619, 517. 
canaliculatum, 520, 628, 
contractum, 517. 
cuneiforme, 623, 517. 
curvatum, 517, 519. 
mickleboroughi, 519, 517. 
minnesotense, 517, 519. 
rectum, 516, 518, 519. 
saffordi, 514, 517. 
schucherti, 518, 519. 
subnasutum, 518, 519. 
subovale, 581, 517. 
Orthonota, 517, 487. 
