No. 415.] NWORIH-AMERICAN INVERTEBRATES. 589 
otocysts symmetrically distributed about the margin, between the tentacular 
bases; manubrium large and long, extending beyond the velum, and ter- 
minating in a series of fringed, lip-like lobes. Hydroid (?).. 
Obelia Per. & Les. 
Generic characters: Medusa flat and disk-like; marginal tentacles 
numerous, projecting slightly inward at the base; otocysts eight, borne on 
base of tentacles at the inner portion; manu- 
brium short and somewhat quadrate. Medusae 
often swimming with everted bell. 
O. geniculata Linn. (Fic. 55). 
Medusa flat; marginal tentacles twenty-four 
at liberation; gonads as oval bodies beneath 
middle of radial canals. Hydroid, — 72:4. 
O. longissima Pallas. 
Fic. 55. — Obelia geniculata Linn. 
Very similar to preceding. Hydroid, — 7/4. 
O. flabellata Hincks. 
(Eucope polygena A. Ag.) 
In general aspects indistinguishable from the preceding species. Hydroid, 
Ibid. . 
O. gelatinosa Pallas. 
(Laomedia gigantea A. Ag.) 
In general feature similar to former, but with only sixteen tentacles at 
liberation, Hydroid, — Z72zg. 
O. dichotoma Linn. 
Indistinguishable from O. gelatinosa. Hydroid, — Z5id. 
O. commisuralis McCr. 
In general features very like the preceding species, but with tentacles 
somewhat more slender and elongated. Hydroid,— 74. 
Tiaropsis diademata Ag. (FIG. 56). 
Medusa ovoid when young, becoming hemispherical at maturity ; radial 
canals four ; marginal tentacles very numerous, the larger with swollen bulbous 
bases ; otocysts eight, situated between bases of tentacles and with otoliths 
in form of arc; gonads extending beneath radial canals; manubrium short, 
With terminal lobes complexly fimbriated. Hydroid (?) 
