C. E. Beecher — Development of the Braoliiopoda. 153 



plates. King. 14 the author of the genus, states that 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 nostologic development among the 

 terebratuloids. The species are smooth, or pauciplicate, and 

 small ; deltidial plates obsolescent, loop more or less unde- 

 veloped. In C. neapolitcma, the lamellse 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, 

 punctas few and large, all of which features are strictly nepi- 

 onic. Besides Cistella and Gwynia, other loop-bearing genera 

 present nostologic features of importance in a natural classifi- 

 cation. 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 

 Terebratulidse, Kraussina and Platydia may be mentioned as 

 belonging to geratologous types with a nostologic tendency. 

 Likewise, in other groups, Atretia in the Rhynchonelliclae, and 

 Strophalosia and Aulosteges in the Productidee, are examples 

 of nostologic types. 



Cistella and Gwynia 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 posteriorly 

 directed mantle lobes, and bundies of setae from body segment. 



Typembryo. — Larva with mantle lobes folded anteriorly over 

 head segment. 



Phylembryo. — Brachiopod covered by protegulum, tentacles of 

 arms developed, bundles of setse dehisced, definition of stomach 

 and oesophagus, direct transformation of larval muscles into 

 those corresponding to muscles of adult animal. 



Deltidhim. — A single plate developed at an early period by 

 the body and pedicle of animal posterior to dorsal hinge, and 

 later ankylosed to ventral valve. 



Deltidial plates. — A nealogic and adult feature produced by 

 the extensions of the ventral mantle lobe into the delthyrium. 



Brachiopoda. — Retrogressive in loss of anal opening and eyes, 

 progressive in concentration of posterior elements, expansion of 

 anterior elements, and limitation of pedicle opening to one valve. 



