272 STUDIES IN EVOLUTION 



Phylembryo. -Bachiopod covered by protegulum, tentacles 

 of arms developed, bundles of setae dehisced, definition of 

 stomach and oesophagus, direct transformation of larval muscles 

 into those corresponding to muscles of adult animal. 



Deltidium. 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 neanic and adult feature produced by 

 the extensions of the ventral mantle lobe into the delthy- 

 rium. 



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. 



Nepionic Period. Young shells before the appearance of 

 distinctive specific characters. 



Neanic Period. Progressive development of the specific 

 features which reach their complete growth in the adult. 



Ephebic Period. Normal adult condition. 



Gerontic Period. Special manifestations of old age in ontog- 

 eny and in phylogeny. 



Paragerontic Types. Extremes of geratology represented by 

 Cistella, Gwynia, and Atretia. 



References. 



1. Beecher, C. E., 1891. Development of the Brachiopoda. Part I. 



Introduction. Amer. Jour. Sci. (3), vol. xli, April. 



2. 1891. Development of Bilobites. Amer. Jour. Sci. (3), vol. 



xlii, July. 



3. and Clarke, J. M., 1889. The Development of some Silurian 



Brachiopoda. Mem. N. Y. State Mus., vol. i. No. 1. 



4. Brooks, W. K., 1879. The Development of Lingula and the Sys- 



tematic Position of the Brachiopoda. Johns Hopkins Univ., Chesa- 

 peake Zool. Lab., Sci. Results, Session of 1878. 



5. Davidson, T., 1851-1885. A Monograph of the British Fossil 



Brachiopoda. Pal. Soc., London. 



6. Deslongchamps, E., 1862. Note sur le developpement du deltidium 



chez les brachiopodes articuls. Butt. Soc. Geol. France (2), t. 

 xix. 



