FREE-SWIMMING COPEPODS OF THE WOODS HOLE REGION. 
163 
E 1. Surfaces of rami of second to fourth pairs of feet without larger spines; appendages of anterior 
maxilliped bristle-shaped E 2 
Surfaces of outer ramus and especially of the second and third joints of the inner ramus of the 
third and fourth pairs of feet armed with stout spines ; distal bristles of the anterior maxilli- 
peds in part soft, vermiform or penicillate E 7 (a, 6) 
E 2. Second and third foot differing from fourth as follows: Basal joints and outer ramus broader 
and stouter, distal edge of second basal joint notched on posterior surface; terminal bristle 
(especially on third foot) with broader margin * Clausocalanus 
Second and third foot not differing from fourth in above-mentioned particulars E 3 
E 3. Outer marginal spines of second and third joints of outer ramus in fourth foot pectinate, lying in 
deep notches Ctenocalanus 
Outer marginal spines of outer rami of feet of usual form E 4 
E 4. Abdomen of 4 segments; its first segment with genital orifice on convex ventral surface, longer 
than the succeeding segments; fifth pair of feet symmetrical or wanting E 4 ? 
Abdomen of 5 segments ; its first segment with genital orifice on left side, shorter than next 
following segments; fifth pair of feet asymmetrical E 4 $ 
E 4 2 On the antero-dorsal surface of head there is a median spine pointing forward; lateral por- 
tions of last thoracic segment prolonged on either side into a powerful point Gaiitanus 2 
Head without a median spine; lateral portions of last thoracic segment rounded or at most 
(on one side) with a short point E 5 2 
E 5 2 Fifth pair of feet 2-jointed, terminating in a stout, curved, denticulate bristle E 6 2 
Fifth pair of feet wanting Pseudocalanus 2 
E 6 2 Rostrum ending in two flaccid threads; last thoracic and genital segment symmetrical ; first 
joint of outer ramus of first foot with external bristle; terminal bristle of fifth foot much 
longer than the two joints of the same taken together Drepanopus 2 
Rostrum wanting; last thoracic and genital segment asymmetrical; first joint of outer ramus 
of first foot without external bristle; terminal bristle of fifth foot about as long as the two 
joints of same taken together Mabianus 2 
E 4 j Feet of fifth pair, especially swollen left one, with a considerable number of peculiarly 
shaped appendages at its tip Mcebianus $ 
Feet of fifth pair long and slender, stylet-shaped Pseudocalanus J 
Feet of fifth pair short, right one with uncinate terminal joint Drepanopus $ 
E7(«). Outer ramus of posterior antenna more than 1^ times as long as inner ramus; cephalothorax 
globose in the female, and stout even in male (fifth pair of feet wanting in the female; in 
the-male there are two uniramous feet) Phaenna 
Outer ramus of posterior antenna less than 14 times as long as inner ramus; cephalothorax 
elliptical E 8(a) 
E8(a). Surfaces (especially posterior surface) of the second and third joints of outer ramus of second 
(and fourth) foot armed with groups of small points; second joint of outer ramus of fourth 
foot without lamellae; fifth foot of female wanting or 1 or 2 jointed; not denticulate on 
inner margin; female with well-developed right and 1-jointed inner ramus in left fifth 
foot Scolecithrix 
Surfaces of outer ramus of second (and third) foot naked ; second joint of outer ramus of 
fourth foot on posterior surface with a series of delicate lamellm; fifth foot of female 2 
or 3 jointed, denticulate on inner margin of the first joint; male with uniramous left and 
lacking right fifth foot Xantliocalanus 
E 7(6). Twenty-fifth joint of anterior antenna fused with twenty-fourth Scolecithrix 
Twenty-fifth joint of anterior antenna separate from the twenty-fourth E 8(6) 
E 8(6). Fifth pair of feet wanting in female and consisting of two uniramous feet in male.. Pliaenna. 
Fifth pair of feet 2 or 3-jointed on either side in female; in male reduced to a single foot 
(the left) Xantliocalanus 
F. Abdomen of 4 segments; its first segment (with the genital opening on the convex ventral surface 
which is often provided with protuberances) at least as long as the following and longer 
than its last segment; fifth pair of feet wanting; mouth parts well developed F2 
Abdomen of 5 segments (with very short, concealed, anal segment), its first segment with genital 
opening on left side, shorter than the next succeeding segments; fifth pair of feet asymmet- 
rical ; blade of mandible, anterior maxilliped, and in part also the maxilla, obsolete F d 
