lxxxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 895, 



when there lived (if we regard the relative size of most Crustacea, 

 and especially that of the living Nebalia) gigantic forms. Such was 

 the Silurian Ceratiocaris ludensis, which was probably more than 

 2 feet in length. 



The modern Nebalia is extremely small, about i inch in length, 1 

 with the body compressed, and the carapace bivalved, as in Limnadia, 

 one of the genuine phyllopods. There is a large movable rostrum 

 overhanging the head ; stalked eyes ; the cephalic portion carries 

 two pairs of antenna? and three pairs of special mouth-organs 

 (mandibles and maxillae) ; the thoracic segments bear eight pairs of 

 short, leaf-like respiratory-feet, which are followed by six pairs of 

 (abdominal) simple swimming-feet, four being large and two rudi- 

 mentary, while the last two segments (7th and 8th) are destitute 

 of appendages, the body terminating in an elongated phyllopod-like 

 caudal fork. Compared with Nebalia, the fossil forms give evidence 

 of an articulated rostrum ; traces of antennae ; the presence of a 

 pair of strong mandibles ; of a large expanded shield in some, and 

 of a folded or bivalved carapace in others ; of the presence of 

 seven or eight body- segments, sometimes carrying branchigerous 

 appendages, the terminal segment carrying a central caudal spine 

 (telson) and two lateral shorter ones (stylets). 



It seems highly probable that the old giant pod-shrimps {Cera- 

 tiocaris, Dithyrocaris, etc.), whose remains occur in the Palaeozoic 

 rocks from the Cambrian to the Carboniferous, are represented by the 

 minute living Nebalia, and that these early forms may have given 

 rise to, and have been the forerunners of, the modern Malacostraca. 

 " In Nebalia," says Claus, " we probably have to do with an offshoot 

 of the phyllopod-like ancestors of the malacostraca, which has 

 persisted on to the present time." 



Turning to the other phyllopodous entomostraca : so far as the 

 evidence borne by the carapace can be accepted, the genus Estheria 

 existed in the fresh and brackish waters of the Devonian Period, in 

 Livonia, Caithness, and Orkney, and also in Nova Scotia and Scotland. 

 It nourished in the European area at several of the Upper Carboni- 

 ferous stages, and was well represented in the Secondary and Tertiary 

 rocks ; it is also living to-day and has a world-wide distribution. 



The Phyllocarida seem in some cases to afford examples of per- 

 sistency of type, and in others of local or temporary specialization. 

 One of the oldest known is the Cambrian Hymenocaris, a prototype 



1 A newly-described species, Nebaliopsis typiea, Sars ; measures as much aa 

 If inch. 



