The taxon
Chydorus faviformis
, described by Birge from North America in 1893, has been considered to occur also in Asia, Australia, and South America. However, careful study of populations from these regions has revealed that all represent different species, none of which is closely related to
C. faviformis
. The taxa described here are
C. obscurirostris
and
C. opacus
from Australia,
C. obscurirostris tasekberae
from Malaysia,
C. sinensis
from China,
C.angustirostris
from India, and
C. parvireticulatus
from South America. The taxon in Malaysia differs somewhat from the corresponding taxon in Australia, but cannot be characterized more closely until males and ephippial females become available. The taxa differ among themselves in number of meshes on the shell of parthenogenetic females, surface patterning within the meshes, shape of the rostrum and height of the mesh walls along the edge and near the tip of the rostrum, stoutness and length of the major seta on the inner distal lobe of trunklimb I, shape of the labral plate, and shape and armament of the postabdomen. Ephippial females all have a single resting egg. They differ in the extent of secondary thickenings of the surface network within the shell meshes and in the amount of pigment deposited in the region of the egg locule. Males are most important for separating the taxa, indicating how necessary they are for working out evolutionary similarities and differences. Unfortunately no males of the taxa from Malaysia, India, and South America have been available. For the others,
C.faviformis
sens. str. is unique in that it is the only taxon in which the male loses its honeycomb (that is, the raised walls of the meshes) on reaching maturity. It also has a sharp pre-anal angle and a marked narrowing of the postabdomen distad from here, which is the pattern typical of species in the
Chydorus sphaericus
complex. None of the other
faviformis
-like species share this characteristic. Because of the marked differences in morphology and in geographical distribution of the species in North America and in South America, it is certain that even during the glacial ages, when the northern
C. faviformis
would have been displaced farthest southward, there was no exchange of either taxon to the other continent. The taxon from Manáos, Brazil listed as
C.faviformis
in the Birge collection is the
C. parvireticulatus
reported from much farther south in Brazil and Argentina. In Australia and Asia, except for the uncertain distinctness of the taxon in Malaysia, all the other taxa are markedly separate from each other and hence give no evidence for transfer, as by resting eggs, between continents or even from one region to another on the same continent. All the taxa have been stable in their geographical occurrence for very long periods of time. In addition to the
faviformis
-like taxa present as distinct species in different regions or on different continents, there are many other species groups of chydorids that have different member species on each continent. One possible explanation of this similarity in gross morphology without any long-distance dispersal of resting eggs to accomplish it is that the various protospecies (corresponding to the species groups) had largely evolved before the original land mass broke up into the present continents and subcontinents. As the distances between the continents increased, the salt-water gaps would come to be impassable barriers to dispersal. Evolution of the isolates would then yield new species, all retaining m any of the features of the protospecies. Each such group from a single protospecies would form the species groups we are now just beginning to recognize.
Reassessing the taxonomic diversity of Plagiothecium section Orthophyllum in the North American bryoflora
