242 STUDIES IN EVOLUTION 



plates are developed in the majority of species. These plates 

 at maturity may entirely limit the pedicle-opening below, so 

 that the pedicle emerges immediately under the beak, or 

 encroaches upon the substance of the beak itself. This type 

 of opening is shown by Zygospira, Spirifer, Rhyndionella, 

 Terebratulina, Magellania^ etc. 



The only divisions of the class which have had continued 

 recognition are the Arthropomata and Lyopomata, proposed 

 by Owen in 1858.* Subsequently various authors gave names 

 to express other characters, but all included the same ele- 

 ments in the two divisions. Professor Huxley's terms, the 

 Articulata and Inarticulata, have also come into current use, 

 and are convenient to express the nature of the union of the 

 valves. All the names proposed for these divisions by Owen, 

 Bronn, Huxley, Gill, and King, are based upon (1) the in- 

 testinal canal whether ending in an anus or in a blind sac, 

 (2) the relative proportions of the viscera and brachia to the 

 shell cavity, and (3) the character of the union of the valves. 



If, as Agassiz has said,f orders should be founded upon 

 facts of development or embryology, the ordinal division 

 into groups expressing the genesis of an important common 

 character should furnish a satisfactory classification. The 

 Articulata and Inarticulata do not appear to have a primary 

 developmental basis in nature. These names may be con- 

 veniently retained as two divisions or sub-classes, but they 

 fail to express the true relationships of the various groups 

 included in them. 



In 1883 Dr. Waagen (Palseontologia Indica) proposed a 

 classification comprising six sub-orders, founded partly on the 

 pedicle-opening and on the form of the brachial supports. 

 Two of his groups, the Mesokaulia and Aphaneropegmata, 

 are nearly equivalent in extent to the Atremata and Pro- 

 tremata now proposed. Daikaulia and Gasteropegmata of 

 Waagen are here included in the Neotremata, and the Telo- 



* Encycl. Brit., 8th ed., XV, 301, 1858. 



t Methods of Study in Natural History, L. Agassiz, 8th ed., 76, 1873. 



