DEVELOPMENT OF THE BRACHIOPODA 271 



the labial appendages are attached directly to the shell, and 

 not to a loop, as in other genera of the family. Cistella may 

 be taken as a representative of paragerontic development 

 among the terebratuloids. The species are smooth or pau- 

 ciplicate, and small; deltidial plates obsolescent, loop more 

 or less undeveloped. In 0. neapolitana the lamellae of the 

 loop are nearly obsolete and are free only near the crura, 

 while the anterior portions are confluent with the valve 

 (Shipley). A slight progression of these reversions would 

 naturally result in a degenerate form like Gwynia, which is, 

 without a calcareous loop; with no surface ornamentation; 

 deltidial plates absent; punctse few and large, all of which 

 features are strictly nepionic. Besides Cistella and Gwynia, 

 other loop-bearing genera present paragerontic features of 

 importance in a natural classification. These consist mainly 

 in their small size; the absence of surface ornaments; the 

 obsolescence of deltidial plates, and the loss of a complete 

 loop supporting the arms. In the Terebratulidaa Kraussina 

 and Platydia may be mentioned as belonging to gerontic 

 types with a paragerontic tendency. Likewise, in other 

 groups, Atretia in the Rhynchonellidse, and Strophalosia and 

 Aulosteges in the Productidas, are examples of paragerontic 

 types. 



Cistella and G-wynia among the genera of brachiopods, 

 therefore, bear the same relation to the terebratuloids that 

 Baculites among the cephalopods bears to the ammonoids. 



Synopsis. 



Protembryo. Ovum and segmented stages before formation 

 of blastula cavity. 



Mesembryo. Blastosphere. 



Metembryo. Gastrula. 



Neoembryo. Trochosphere and cephalula, with posteri- 

 orly directed mantle lobes, and bundles of setae from body 

 segment. 



Typembryo. Larva with mantle lobes folded anteriorly over 

 head segment. 



