358 CRUSTACEA. 



abdominal legs of the female (many Macrura) or enclosed be- 

 tween the flexed abdomen and the thorax. 



Fossil forms. The differentiation of the main types of Crus- 

 tacea had occurred before the palaeontological record begins. 

 The Carboniferous rocks contain examples of four out of the 

 five Entomostracan orders, the Branchiopods being represented 

 among other forms by Estheria, a genus which has existed from 

 the Devonian period to the present day, the Cirripedes by 

 Pollicipes and Scalpellum, which go back to the Ordovician, 

 and the Ostracods by a variety of genera which already peopled 

 the Cambrian waters, the genera Cythere and Bairdia extending 

 from the Ordovician to the present day. The fourth order, 

 the Trilobites, is well established in the Cambrian rocks, attains 

 its highest development in the Ordovician and by Devonian 

 times was already on the wane. It is doubtful if they extended 

 beyond the Carboniferous period. The remaining Entomostra- 

 can order, the Copepods, consisting of small and thin-shelled 

 forms, have not been recognized in any geological formation. 



A number of genera found in th~ Palaeozoic rocks from the 

 Cambrian to the Carboniferous appear to be related to the 

 most primitive group of the Malacostraca Nebalia and its 

 allies. Among these may be mentioned Ceratiocaris (Cambrian 

 to Carboniferous) and Hymenocaris (Cambrian), Echinocaris 

 (Devonian) and Dithyrocaris (Carboniferous). They have thin 

 bivalved or at least bilobed shells and the abdomen is apparently 

 without appendages though ending in a well-marked caudal 

 fork. They have been referred by many authors to the Phyllo- 

 pods, but the presence in Ceratiocaris of a rostral plate appears 

 to justify the association of this genus at least with Nebalia. 



The Carboniferous genera Palaeocaris and Acanthotelson and 

 the Permian Gampsonyx and Nectotelson (Brocchi) and perhaps 

 also Gasocaris (Fritsch) find their ally, as was first ably shown 

 by Caiman, in the existing Tasmanian genus Anaspides, and 

 form, with it, the group of the Syncarida (Packard). 



The caridoid type of Malacostraca shrimp-like forms with 

 dorsal shield, well-developed swimming abdomen ending in a 

 caudal fin, stalked eyes and a scale-like exopodite to the 2nd 

 antenna is represented in the Carboniferous rocks by a number 

 of well-preserved fossils : Palaeopalaemon Whitfield, appearing 

 in the Devonian, Crangopsis (Palaeocrangon) and Anthrapalae- 



