BRACHIOPODA. 185 



II. 



BRACHIOPODA ARTI.CULATA. 



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 iNAKTicnLATA and erachiopoda akticulata is dei-ived from the 

 structure of ^analogous living- foi'ms. 



