i 9 2 DISCOVERY REPORTS 



It is obvious that this must be so, for if a hinge is functional its axle must be straight, 

 or approximately so, and only a short piece of the oval outline of a Cyprid can be 

 considered as approximately straight. If the fusion between the two valves is relatively 

 great, as in the Halocyprids, then it is found that the dorsal side has become flattened 

 out to produce a long straight axis for the hinge. In Gigantocypris , however, there is 

 no such dorsal flattening — the outline, when seen from the side, being in fact almost 

 circular — and yet the fusion between the two valves extends more than half-way round 

 the circumference (Fig. i). This is the character which was used by Muller (1912, p. 9) 

 for distinguishing Gigantocypris from all other members of the subfamily Cypridininae, 

 to which it belongs, and it might equally well have been used to distinguish it from all 

 other Ostracods. The fusion between the two valves is complete and is marked by 

 a definite line (Plate XXXIX, figs. 9, 11), but this represents merely a slight thickening 

 of the chitin and no line of flexure. 



It may thus be stated that the body of Gigantocypris is enclosed in an almost spherical 

 shell provided with a ventral slit, which represents the relatively short length over which 

 the valves are not fused. The slit is enlarged at both ends, anteriorly as the "rostral 

 incisur" (Plate XXXIX, figs. 6-8) and posteriorly as an oval opening (Plate XXXIX, 

 figs. 9-11) which Muller states (1895, p. 156) is absent in other Cypridinids. In between, 

 the edges of the valves are close together, and there is no evidence that the narrow slit 

 between them can be widened in any way. From the antennal notches, which together 

 form the " rostral incisur", back as far as the base of the caudal furca, the edges of 

 the valves are wedge-shaped and merely come together closely, but behind this region 

 and back to the posterior oval opening they are asymmetrical and in section appear to 

 fit into each other, thus producing a very effective closure. 



The anterior enlargement of the ventral slit consists of a median prolongation forwards 

 (Plate XXXIX, fig. 6) from the two lateral antennal notches. This median part is never 

 really closed, but its margins are heavily armed with setules. It always forms a narrow 

 space through which the antennules can be projected (Plate XXXIX, fig. 7). The antennal 

 notches, as in the majority of the Myodocopa, serve as rowlocks for the swimming 

 organs, the exopodites of the antennae (Plate XXXIX, fig. 3). The long basal joint of 

 the exopodite presses close against the base of the emargination (Plate XXXIX, fig. 8), 

 and, as this joint is relatively slender, it does not completely fill the antennal notch. 

 Consequently, where the mouths of these notches meet, there is a small median opening. 

 This is the only effective entrance into the cavity of the spherical shell. It is through 

 this aperture that all food and water for respiration must be taken in. 



For the further comparison of the body and limbs of Gigantocypris with those of 

 a typical Cypridinid the important point to be grasped is that, while the carapace has 

 enlarged to such a great extent, the limbs have neither enlarged proportionately, nor 

 have they become spaced out (Fig. 1 a, b). They remain closely packed together, having 

 the same interrelations as in Doloria and occupy only a relatively small antero-ventral 

 region of the whole organism. As a result, the region posterior to the limbs has become 

 an enormous hemisphere (Plate XXXIX, fig. 1), while the region in front, that is, between 



