Evolution of the Paleozoic Braehiojpoda. — Crane. 403 
muscular implantation. Gradual excavation of this solid plate ensued, 
and the formation of a more or less vaulted platform, extremely devel- 
oped, in the feebly articulated trimerellids of those Silurian seas, which 
favored the rapid development of the platform-bearing Brachiopoda, 
a race which was abruptly exterminated at the close of the Niagara and 
Wenlock period. Hall's new genus Barroisella is a divergent so marked 
by the development of deltidial callosities as to indicate their approxi- 
mating specialization for articulating and interlocking purposes. Thus 
we get most striking evidence of a tendency to span the interval between 
the so-called edentulous Inarticulata and the articulated genera in the 
Linguloid and Trimerelloid groups. 
The genus Obolus is shown to be more specialized than Obolella, less 
so than Llngula, Neohohis being an intermediate form with cardinal 
processes, also indicative of progress in this direction towards the 
At'tlculata. In Oholus, however, the muscular scars are excavated as 
in Lingula, not elevated as in the forms tending to Trimerella. Thus 
we get indications in the history of the ancestral Trimerellids of the 
attainment of a like remarkable resultant along distinct lines of devel- 
opment, of which another instance has been furnished by Messrs. 
Fischer and Oehlert's recent studies of the development of the living 
Magellance of the boreal and austral oceans, to which we had elsewhere 
occasion to refer.* As Hall and Clarke's generalizations are formulated 
with a due regard to geological sequence, they possess more validity 
than the phylogenetic deductions enunciated by a Teutonic paleontol- 
ogist,in which that important factor was somewhat neglected. f "We 
have yet to seek," the American brachiopodists conclude, "the source 
whence these numerous closely allied primordial groups are derived, in 
some earlier comprehensive stock of which we have yet no knowledge. 
The ages preceding the Silurian afforded abundant time for a tendency 
to variability to express itself" (p. 168). 
From this satisfactory discussion of the origin and development of 
the paleozoic unarticulated genera and species, Hall and Clarke proceed 
to consider the structure and relations of the far more numerous and 
more complicated order of the articulated species, and commence with 
the Ortlaoids, the lowest forms of the Articulata, as, by common con- 
S3nt, they are now regarded. The allied strophomenoids, streptorhyn- 
choids, and leptaenoids, as defined by Dalman, are then treated of and 
the first part terminates with a discussion of some Carboniferous pro- 
ductoids. The spire-bearers, rhynchonelloids and terebratuloids, of the 
Paleozoic seas are thus left for the concluding volume, when we may 
look for a valuable general summary of results and for that systematic 
classification, based on their completed investigations, which the authors 
are bound to propose in the interests of students for the root, stem, 
branches, and twigs of the genealogical tree of the Brachiopoda, as 
they have definitely abandoned the family names hitherto in vogue. It 
*0n the Distribution and Generic Evolution of some recent Brach- 
iopoda, by Agnes Crane, Natural Science, January, 1893. 
fNeumayr, "Die Stiimme des Thierreichs Brachiopoda," 1890. 
