194 Proeeedingf of the Royal Irish Academii. 



therefore conjecture tbat some special conditions favourable to its develop- 

 ment were present at this spot, and the stratigraphical relations indicate that 

 some such special conditions might have been produced as a consequence of 

 the rec-ent retreat of the sea and the development of fresh or possibly brackish 

 water lagoons along the margin of the sand-bank. 



HI. — Slmmakt of Piikviocsly Described Gexeka. 



Tlie order Conchostraca is divided into two families — the Limnadiidae and 

 the Limuetidae. 



Limnetis, the sole genus of the Limnetidae, is simpler in structure, 

 particularly as regaitis the tail-segment and the reduced number of trunk- 

 liml-s, than the . ' the Limnadiidae. It differs from all these, with the 



exception of Cy,. : ,,of which the position is uncertain, in the possession 



of only one pair of claspers in the male. 



Cyclestlieria resembles limnetis in this respect, but in others is much 

 more closely '' ' ;.i and the othor Limnadiidae. 



Thedes'.: , lug Conchostracan genera are scaltei-ed through 



a variety of publications. In order to give an idea of the relationships of the 

 fossils described, it has been thought ad\'isable to include here the following 

 synopsis of generic characters : — 



I. — LiMXADU : 



(1) i>\ib\\ large, oval, greatly compressed, very thin, smooth, wiih about 



IH lined of growth. 



(2) Uead small, but furnished with haft-organ. 



(3) First pair of aiileuuae, or antenuules. couiparaUvcly short. 



(4) > iir of aiilenaae ^^ - r than outer, but both 



into numerous la:. u inner edge with long 



natatory bristles, on outer with short spines. 

 1 5) About 24 pairs of tmnk-limbs. 



(0) In males two anterior pairs of irunk-limbs are modified as clasping organs. 

 (7) Caudal lamellae drawn out below to a sharp, not claw-like, augle, and 



finely dentated posteriorly. 

 II. — EcLuccADU : 



(1) Shell narrow oblong, with 4-a lines of growth. 

 {2) Head similar to Limnadia. Uaf t-organ present. 



(3 & 4) Antennae do not differ essentially from those of Limnadia, but the 



gills are larger. 

 (a) About la pairs of trunk-limbs. 



(6) In males two anterior pairs of trunk-Umbs are modified as clasping organs. 



(7) Telson similar to that of Limnadia. 



lUmark*. — Eohmnadia is closely allied to Limnadia, the only essential 

 difference being that Uae species of Eulimnadia, like those of the genus Estheria, 

 are bi-5«xual, while males of the genub Limnadia have not yet beien found. 



