THARYBIDAK FROM THE GULF OF MEXICO 349 



SYNOPTIC KEY TO THE GENERA OF THARYBIDAE 



SrRincnts 24 and 25 of first antennae fused: mandibular gnathal lobe with 3-4 nionocuspidate spiniform teeth; female 

 fifth legs with 2 segments; male right fifth leg with conspicuous endopodite; external exopodal spines of legs 2-4 

 bearing marginal denticles Parundinella 



Segments 24 and 25 of first antennae separate; mandibular gnathal lobe with more than 4 monocuspidate spiniform 

 teeth; female fifth legs with 3 segments; male right fifth leg with endopodite vestigial or lacking; external exopodal 

 spines of legs 2-4 not ornamented or bearing fine hairs 2 



Thoracic segment I and cephalon partially separated; segments IV andV fused or separated; rostrum with bifid basal 

 portion; first maxillae with inner lobe 1 not of greater surface area than remaining lobes and rami, endopodite 

 extending to or beyond distal border of inner lobe 1, exopodite with 2-3 setae; maxillipeds with segment 2 of uni- 

 form thickness; female fifth legs with segments more or less equal in length; terminal spine of legs 2-4 with fine 

 shallow serrations; lobes of second maxillae crowded within distal half of appendage Undinella 



Thoracic segment I fused with cephalon, segments IV and V fused; rostrum with rounded knob-like basal portion 

 bearing 2 long filaments; first maxillae with inner lobe 1 large, extending beyond remainer of appendage, exopodite 

 with 5 setae; maxillipeds with segment 2 swollen, greatest thickness at midlength; female fifth legs with terminal 

 segment longer than preceding 2; terminal spine of legs 2-4 with coarse deep serrations; lobes of second maxillae 

 extend over more than half of appendage Tharybis 



It is noteworthy that several of the characters 

 used in separating the 3 genera tend to intergrade 

 as follows: (1) ratio of surface area of inner lobe 

 1 to remaining lobes and rami of first maxillae 

 (largest in Tharybis; less in Undinella; smallest in 

 Parundinella); (2) segmentation of female fifth legs 

 (3 segments of about equal length in Undinella; 

 basal and mid-segments considerably reduced in 

 Thanjbix; loss of 1 segment, presumably the second, 

 and reduction of basal segment in Parundinella); 

 (3) segments 24 and 25 of the first antennae (separ- 

 ate in Tharybis and Undinella; fused in Parundi- 

 nella spinodenticula, new species, but partially 

 separated in P. manicula, new species). 



Parundinella spinodenticula, new species 



Plate 1, Fir.s. 1-16; pi. 2, Firs. 1-11. 



Localities, Malerial.~GuU of Mexico: lat. 22° 20' X., 

 long. 87° 31' W., (Alaska, cruise 4, station 11, 13 January 

 1952, 1 meter depth of plankton tow; 2 adult females, 1 

 intersexed individual); lat, 23° 18' N., long. 97° 38' W., 

 (Alaska, cruise 2, station 11, 7 June 1951, 1 m, depth of 

 plankton tow; 1 adult male). 



Measurements. — Specimens measured in dorsal 

 view. Length of cephalothorax along midsagittal 

 plane from anteriormost limit of forehead to post- 

 erior margin of intersegmental membrane between 

 thoracic fusion segment IV-V and genital seg- 

 ment; length of abdomen from anterior margin of 

 genital segment to posteriormost limit of right 

 furcal ramus. Measurements made at lOO.x mag- 

 nification with aid of ocular micrometer; speci- 

 mens immersed in aqueous solution of 50 percent 

 glycerine. Slender glass rods used to sii])port 

 cephalothorax and abdomen in horizontal position 

 during measurements of each specimen. 



Measurements given include total length (TL) 

 and cephalothorax-abdomen length ratio (CAR). 



1. Adult Fem.\le: (holotype) TL 0.84 mm., CAR 3.1:1; 

 (paratypes) TL 0.83 mm.; TL 0.84 mm. 



2. Adult M.\le: TL 0.74 mm.; CAR 2.7:1. 



Description. — A minute tharybid characterized 

 at low magnifications by a robust ovate cephalo- 

 tliorax with first antennae extending to thoracic 

 fusion segment IV-V and a short slender abdomen 

 about half as long as the fourth pair of swimming 

 legs. 



Adult Fem.ale: Cephalothorax robust, greatest 

 depth between maxillipeds and first legs; forehead 

 strongly vaulted (pi. 1, figs. 3, 5; pi. 2, fig. 11). 

 Rostrum with distal half of basal portion bifid, 

 each process bearing a short rostral filament (pi. 1 , 

 fig. 8). Terminal portions of thoracic fusion 

 segment IV-V in lateral view produced in a tri- 

 angular lappet with rounded apex, extending pos- 

 teriad beyond midlength of genital segment (pi. 1, 

 figs. 3, 5). 



Genital segment in dorsal view with moderate 

 lateral swellings confined to anterior three-fourths 

 of segment; segment narrowed abruptly just 

 posterior to seminal receptacles (pi. 1, figs. 5, 16). 

 Four abdominal segments and furcal rami with 

 proportional lengths taken along midsagittal plane 

 of 37, 19, 19, 8, 17 (=100). Furcal rami appro.x- 

 imately twice as long as broad, each bearing 6 

 setae; 4 most terminal setae elongated, almost 

 equal in length; lateral and medial setae shorter 

 than ramus, unequal in length (pi. 1, fig. 5). 



Cephalic appendages differing in several details 

 from corresponding ap|)endages of Tharybis. 

 First antennae (pi. 1, fig. 4) with 23 visible seg- 



