BRA CHIOPODA. 
185 
II. 
BRACHIOPOD A ARTICUL AT A. 
Valves articulated; intestine terminating in a blind sac;* shell-substance essentially 
calcareous. 
So far as our knowledge of the fossil species extends, there are very few of 
the forms included in this division to which the above definition does not apply. 
Several genera show very considerable modification of the articulating appa¬ 
ratus, but with rare exceptions these modifications in each group appear to be 
progressive, extending along certain lines of development and finally acquiring 
an extravagant manifestation, which may terminate abruptly or result in the 
degeneration and obsolescence of some of the parts. 
In the group of fossils which are currently referred to Orthis there are 
several well marked subdivisions, which are in some degree coincident with 
geological succession, and the later of these show a tendency to the extravagant 
development of certain characteristic features, while the genus Enteletes, 
which can be affiliated only with Orthis, presents a most remarkable exaggera¬ 
tion in the development of certain parts. 
On the other hand, the articulating apparatus in Strophodonta, Productus, 
etc., is by some authors regarded as being in a degenerative condition, and on 
this account they would place these genera near the base of the articulate 
division, though the type of structure exhibited by them is much more highly 
modified than in Orthis and its immediate allies in the older rocks. 
The mode of articulation in Productus appears to have come legitimately 
from the progress and development of some portions of the articulating appa- 
* The definition of the brachiopoda inarticulata and brachiopoda articolata is derived from the 
structure of ..analogous living forms. 
