202 Proceedings of the Eoyal Irish A eaiJctini. 



Specimen 16. — Outline of two carapace valves and antennae ; poor. Depth iubore 

 832 feet. 



Specimen 17. — Shell only ; 4 mm. long. Identical with fipecimen 19. Depth in 

 bore, 831 feet. 



Specimen 18. — Outline of one carapace valve and one antenna. Antennal segments 

 and hairs well shown. Depth in bore, 831 feet. From same horizon as 

 specimen 17. 



Specimen 10. — Shell (6-5 mm.) as sketched in text-tig. 3, and, lying near it, much- 

 crushed animal, with antennae, telson, etc., and outlines of two carapace 

 valves. Depth in bore, alH>ut 830 feet. From same slab as specimens 

 1, 4, 8, 13. 



Specimen 20. — Outline of carapace valves and crushed remains. Depth in bore, 

 about 830 feet. On same slab as specimen 19. 



Specimen 21. — Good .^ihell inipre-^sion (5-5 mm.), identical with specimens 

 19 and 17. Depth in bore, 830 feet. 



Specimen 22. — Narrow Estherian slidl. about G mm. Depth in bore, about 

 830 feel. 



Specimen 23. — " Estherian " shells of approximately same type as specimen 19. 

 Depth in bore, 827 feet. 



\' 1 1 .— «;oKci,USION. 



The cosmopolitan distributinn of most of the Conchi^stracau genera has 

 lieen said by Tackard to indicate their high antiquity. The present fossil 

 rouiains certainly snpport such a conclusion. Tliey are essentially Estherian 

 in character, (lifTering only in the absence of a second pair of claspers in the 

 male, which character allies them to Cyclestheria and Limnetis. Without 

 claiming for them direct ancestry, one can recojpiize in them something 

 closer to the stem from which the divergent gcnem of the order originally 

 sprang. One is foicibly struck, however, with the absence of any niarkcil 

 evolutionary development in the long interval since Carboniferous times. 

 This may possibly be connected with their adaptation to a mode of life very 

 clearly defined by stre.-.s of physical conditions, and affording no outlet for 

 elalwration of stnicture. 



In how far the general pnnciple followed by palaeontologists of reasoning 

 from the habits and surroandings of a li^^ng fortn to those of a closely allied 

 fossil can be applied to the ca.'^? of the Conchostraca must as yet remain in 

 the region of speculation. It might, perhaps, fairly be argued that the 

 appearance of animals of such a high degree of specialise*! a<laptation to 

 peculiar conditions must imply the recurrence throughout geological histor}* 

 of similar conditions in whatever place or period they occur. This, however. 



