PAKASITIC COPEPODA. 203 



FAMILY : DKJHKLKSTIID^E. 

 Caetrodes,* n. gen. 



Body regions distinct. Head covered with a dorsal carapace which is obovate in 

 shape, strongly arched and considerably widened anteriorly, narrower and rounded 

 posteriorly. This posterior portion is flattened and projects far back over the thorax 

 segments, but is not attached to them. Frontal margin turned under the carapace 

 a little, carrying the base of the anterior antennae back with it on the ventral surface. 



At least four (probably five) free thorax segments, indistinctly separated and 

 diminishing in width posteriorly, the fifth one sending back a wide lobe on either side 

 of the genital segment. Genital segment small, transversely oblong, enclosed on 

 three sides by the fifth segment. 



Abdomen small, hemispherical, one-jointed. Anal papillae longer than the abdomen, 

 narrow, cylindrical, and terminating in a spine and a claw. 



First antennae five-jointed, slender, with very few setee except on the terminal 

 joint. Second pair stout, ending in a prehensile claw. Mouth-tube short and wide ; 

 mouth-opening terminal. 



First maxillipeds rudimentary, attached close beside the second maxillae and of 

 about the same size. Second pair slender, two-jointed. Two pairs of biramose 

 swimming legs, close together and just behind the second maxillipeds ; rami linear 

 and two-jointed. Egg-tubes longer than the body ; eggs large and uniseriate. 



Caetrodes pholas,t n. sp. Plate IV., figs. 48 to 57. 



Female. Head wider than the rest of the body and two-fifths of the entire length ; 

 covered dorsally with a strongly arched carapace which is divided into right and left 

 halves by a prominent ridge or rib at the centre. The posterior margin of this 

 carapace is prolonged backward in the form of a thin flattened plate which covers 

 the anterior half of the thorax segments. 



With the point of a needle, or by sharply flexing the body, this plate may be 

 lifted away from the thorax segments, and this shows that it is not attached to them 

 in any way. There is no trace of the median rib in this posterior part of the 

 carapace. The passage from the arched to the flattened portion of the carapace is 

 very irregular and forms a broken line over the posterior margin of the head. At the 

 centre there is a wide triangular sinus extending forward, with its point on the 

 median line. On either side of this is a blunt, rounded projection extending back- 

 ward, outside of which is a wavy line curving forward as it runs toward the margin. 

 There are no traces of frontal plates or of eyes. 



The thorax is composed of at least four (probably five) free segments, which are 

 imperfectly separated from one another. 



* C<ztrodes, like a small round shield. 



t Pholas, lurking in a hole or burrow. 



2 D 2 



