BRANCHIOPODA. 509 



occurring in astonishing abundance. A few forms (such as Bairdia 

 and Pontocypris) are marine in habit. The family of the Cypridce 

 seems to have attained its maximum at the present day, but is rep- 

 resented in deposits as old as the Carboniferous by forms which 

 are believed to be referable to the living genera Candona and 

 Bairdia. Specimens of the Carboniferous genus Palceocypris have 

 been found, showing the eye, the antennas, and the jointed limbs, 

 of which there are two pairs behind the mandibles, the last pair 

 being strongly incurved. 



In the family of the Cytheridce, lastly, the shell is minute, thick, 

 inequivalve, and generally elongated or reniform in shape. The 

 dorsal margin of the carapace has two denticles in the right valve 

 fitting into corresponding sockets in the left valve. The surface is 

 smooth, or is variously ornamented with tubercles or spines. The 

 genera of Cytheridce are mainly Recent and Tertiary ; but the genus 

 Cy there ranges from the Silurian to the present day, and there are 

 various Secondary types belonging to the genera Cythereis and 

 Cytheridea. 



Order II. Copepoda. — Small Crustaceans having bifid natatory 

 feet, and the head and thorax usually covered with a carapace. Two 

 caudal locomotive appendages are often present ; but the abdomen does 

 not carry limbs. Segmentation is distinct in the free forms ; but it is 

 more or less lost in the females of the parasitic types. 



The order of the Copepoda comprises numerous small Crusta- 

 ceans, which inhabit both fresh and salt water, many types being 

 parasitic in habit. No certain examples of the Copepods have 

 hitherto been detected in the fossil condition. 



Division B. Branchiopoda. 



The Crustaceans included in this division have many branchiae, 

 and these are attached to the legs, which are often numerous, and 

 are formed for swimming. In other cases the legs themselves are 

 flattened out so as to form branchiae. The body is either naked, 

 or is protected by a carapace, which may enclose either the entire 

 body, or the head and thorax only. The mouth is provided with 

 organs of mastication. 



The Branchiopoda comprise the Cladocera, the Phyllopoda, the 

 Phyllocarida, and probably the Trilobita, though this last departs 

 in many respects from the first three groups. 



Order I. Cladocera. — The members of this order are small 

 Crustaceans, which have a distinct head, and have the whole of the 

 remaifider of the body enclosed within a bivalved carapace. The feet 

 are few in number {usually four, five, or six pairs), and are mostly 

 respiratory, carrying the bra?ichice. Two pairs of antennae are 



