136 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
spire bearer is a small undescribed form from the Row Park formation of Mary- 
land and a probably contemporaneous one, possibly the same species, from the 
Crown Point formation of New York. Only single specimens are known in each 
instance ; consequently, the interior is not known. Both of these are Protozyga- 
like shells. This seems to be the most primitive of the genera. The spire has only 
one volution. This genus, however, is fairly long ranged and becomes fairly 
large in the late members of the genus in the Trenton group. No originating 
form can now be postulated. 
Idiospira and Cyclospira have more complicated spires. The former suggests 
affinities with Atrypa and allies in which the spire is coiled toward the brachial 
valve. In the latter the spiral coils are directed inward. Cyclospira and Zygospira 
may have arisen from Protozyga. The same origin is possible for /diospira. 
PLECTAMBONITACEA.—The present study shows the members of this super- 
family to be numerous, extensively distributed, and diverse in structure. In North 
America the first of these appear in rocks deposited during late Canadian time, 
and a fairly large development took place in Whiterock time and culminated in 
the Porterfield stage. Three major stocks can be recognized: The palaeostro- 
phomenoids, probably originated from Aporthophyla, the leptellinoids, and the 
sowerbyelloids ; the origin of the last two is obscure. The first group is well rep- 
resented in Europe as well as America. The group also is commonest in rocks 
deposited during Porterfield time. Distinctive genera are Apatomorpha, Palae- 
ostrophomena, Glyptambonites, and Sowerbyites, which have similar growth 
forms to the Strophomenidae. These genera are characterized by having a single 
septum in the brachial valve. The sowerbyelloids have two or more septa and 
form a prolific group that appears in the Whiterock stage and continues on into 
the Silurian. This group may have come from Syndielasma or some still more 
primitive pseudopunctate shell such as Pelonomia. The leptellinoids are charac- 
terized by a visceral disk and strong median septum. They appear in the late 
Canadian with Leptella and become moderately common in the early part of the 
Middle Ordovician. They continue into the Silurian but are generally rare fos- 
sils at that time. It is not possible to say what the derivation of the Plectam- 
bonitids is, but the most likely ultimate origin is from an orthid stock sometime 
in the early Ordovician or late Cambrian. Cymbithyris Cooper (1952), a con- 
cavo-convex billingsellid of the late Cambrian, is a possibility. 
STROPHOMENIDAE.—It has been shown that the resupinate and normally con- 
vex forms of this division are closely related. The resupination of the shells 
seems not to be a valid means of classification (A. Williams, 1953). The genera 
of this subfamily probably originated in some orthoid stock in the early Ordovi- 
cian, possibly from some form like Taffia or Aporthophyla. Kirkina is the oldest 
known form in this group having a lobate cardinal process. At any rate it ap- 
pears in the Marmor stage as Macrocoelia and rapidly becomes more diverse. 
Species of Strophomena and Opikina are the commonest shells in certain parts 
of the country in rocks deposited during the Wilderness stage. 
LEPTAENIDAE.—This division seems to have originated with Hesperinia in the 
high Pogonip, deposited during the Whiterock stage. At any rate this genus has 
