BRAcCHIOPODA. ‘ 917 
The great gulf which has seemed to exist between the Inarticu- 
late or Lyopomatous, and the Articulate or Arthropomatous divi- 
sions of the Class Brachiopoda; those without teeth, and those 
with teeth ; those with a largely corneous shell, and those whose 
shell is essentially calcareous, is not yet fully spanned at many 
points. 
These divisions were based upon the study of living brachiopods 
in which all the characteristic differences are pronounced and 
fixed. Itis natural, however, to find among the early brachiopods, 
in which the adjustment of the organism to its conditions was 
highly sensitive, that the oscillation and specialization of charac- 
ters has been very rapid. The development of articulating 
processes has already been noticed among the linguloids in 
Barrorskyia, Tomasina and Trimere.ia, among the oboloids in 
SponpyLogo: us, and among the siphonotretoids in TrematTozotus. 
It is known that the shell of many inarticulates is almost wholly 
calcareous, asin the TrmerzLiip®, and all of the GasTBROPEGMATA. 
The alteration in the nature of the shell-substance from the pro- 
toconch or its exemplar, Paterna, which appears to be wholly 
or essentially corneous, to the typical articulate brachiopod, in 
which the corneous substance is reduced to a thin epidermal film, 
is a gradual process whose various stages are well understood. In 
Oxortxia, Exxanta, and the early forms of Lrneuna, the deposition 
of calcareous salts in the shell was already advanced, these layers 
alternating with thinner layers of corneous substance. The 
gradual and eventual predominance of the calcareous shell-matter 
along both of these lines of development is seen in the ponderous 
Trimerellids of the later Silurian. The graduation of the corne- 
ous Parerina (Kutorgina Labradorica, var. Swantonensis) through 
Kutorgina Labradorica, and into the true calcareous Kutorginas 
(K. cingulata, K. Whitfieldi), is similar evidence. In Kutorgina 
Latourensis, Maciuew described a minute tooth on either side of 
the pedicle-opening, and it has been stated that A. cingulata 
shows faint traces of articulating processes at or near the extremi- 
ties of the cardinal line. Such cases indicate a direct transgres- 
sion in the texture and composition of the shell from the most 
primitive inarticulate type to the articulate. In this feature only, 
the connection between the two divisions of the class is no closer or 
more clearly manifested than in the instances mentioned, but it 
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