580 Forty-fifth Report on the State Museum. 



II. 



Brachiopoda Articulata. 



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 a,bove 

 definition does not apply. Several genera show very considera- 

 ble modification of the articulating apparatus, but with rare 

 exceptions these modifications in each group appear to be pro- 

 gressive, extending along certain lines of development and finally 

 acquiring an extravagant manifestation, which may terminate 

 abruptly or be followed by the degeneration and obsolescence of 

 some of the parts. 



For example, the articulating apparatus in Stropheodonta, 

 Productus, etc , may be regarded as in a degenerative condition, 

 and on this account these genera are sometimes placed near the 

 base of the articulate division, though the type of structure exhib- 

 ited 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 apparatus on the one hand, and the obsoles- 

 cence of other characters, which appear in the earlier forms of 

 strophomenoid shells, and have become modified in their pro- 

 gress through the chonetids. While this may be regarded as a 

 degeneration, it is also a progressive modification. 



Of the various designations which have been proposed with a 

 signification equivalent to Professor Huxley's term for this sub- 

 division of the Brachiopoda, are Arthropomata, Owen ; Ci.isten- 

 tkrata, King ; while Bronn made use of the two names Testi- 

 cardines and Apygia. 



Orthis, Dalman. 1828. 

 (Plates 9-12.) 

 Synonyms; Uysterolithes, Schlotheim, 1820 ; OrthaniboniteS) 

 Pander, 1830. 



Shell subcircular, subovate or subquadrate in outline. Valves 

 more or less convex, the smaller or brachial valve being some- 

 times nearly Hat or slightlv concave. Hinge-line straight and 

 equal to, or shorter than the greatest width of the shell. Cardi- 



132 



