8 CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 
Order CYDIPPIDZ Gegenbaur, 1856. 
Cydippide, GEGENBAUR, 1856, Archiv fir Naturgesch., Jahrg. 22, p. 196.—CHUN, 
1880, Ctenophoren des Golfes von Neapel, pp. 30, 273, 275-—MosER, 1903, 
Ctenophoren der Siboga-Expedition, p. 4.— VANHOFFEN, 1906, Nordisches 
Plankton, Ctenophoren, XI, p. 2. 
Pleurobrachiide, Ghigi, 1909, Ctenofori, Firenze, vol. 2, fasc, 1, p. 7- 
CHARACTERS OF THE CYDIPPIDA, 
Ctenophore with 2 tentacles in the funnel diameter which arise 
from sheath-like clefts in the sides of the body. The body is usually 
laterally compressed, the tentacular diameter being the wider, although 
it may be spherical or cylindrical. There are neither oral lobes nor 
auricles, The meridional and paragastric canals end blindly. The 
tentacles may be simple or with side branches. The axis of the tentacle 
is mesodermal as in other ctenophores. 
Genus MERTENSIA Lesson, 1836. 
Mertensia, Lesson, 1836, Annals des Sci. Nat., sér. x, tome 5, p. 253; 1843, Hist. 
Zooph. Acal., p. 100.—Mo6rcu, 1857, Nat. Bid. til en Beskriv. af Grénland, 
Bi 97.—Acassiz, A., 1865, North Amer. Acal., p. 26.—CHuN, 1898, Ergeb. der 
lankton-Expedition, Ctenophoren, p. 10.—VANHOFFEN, 1906, Nordisches 
Plankton, 11, Ctenophoren, p. 2.—MosEr, 1909, Ctenophoren der deutsch. 
Stidpolar-Exped., Bd. 11, Zool. 3, p. 123. 
GENERIC CHARACTERS. 
Cydippide in which the body is laterally compressed, the tentacular 
axis being wider than the axis through the plane of the stomodeum. The 
4 subtentacular rows of cilia are longer than the 4 subventral rows and 
arise at a higher level, and are farther removed from the apical sense- 
organ than the latter. There are no wing-like protuberances upon the 
sensory pole-plate. 
The type species is Meriensia ovum of the Arctic Ocean. It was 
first described by Fabricius, 1780, as Beroé ovum. 
Mertensia ovum Lesson. (Fig. 1, plate 1.) 
Beroé ovum, FaBricius, 1780, Fauna Grénlandica, p. 362, No. 355. 
Cydippe ovum, C. cucullus, EscHscHoutz, 1829, Syst. der Acal., p. 25. 
Beroé pileus, ScORESBY, 1820, Arct. Reg., 2, plate 16, fig. 4. 
Mertensia ovum, Lesson, 1836, Annales des Sci. Nat., sér. 2, tome 5, p. 254. 
Cydippe (Mertensia) ovum, M6rcH, 1857, Beskriv. af Grénland, p. 97. 
Mertensia scoresbyi, Lesson, 1843, Hist. Zooph. Acal., p. 100. 
Cydippe cucumis, Lesson, 1843, Hist. Zooph. Acal., p. 105. 
Mertensia cucullus, Acassiz, L., 1860, Cont. Nat. Hist. U. S., vol. 3, p. 293. 
Mertensia ovum, Acassiz, A., 1865, North Amer. Acalephe, p. 26, figs. 29 to 37; 
1874, Mem. American Acad., vol. 10, No. 3, p. 375, figs. 1, 2. —VANHOFFEN, 
1895, Bibliotheca Zoologica, Heft 20, Lig. 1, p. 21.—CHUN, 1898, Cteno- 
phoren der Plankton-Expedition, p. 1o—R6MER, 1903, Fauna Arctica, Cteno- 
Satin Bd. 3, p. 72 (full list of literature) —VANHOFFEN, 1906, Nordisches 
lankton, x1, Ctenophoren, p. 2, figs. 1 to 3.—BicGELow, 1909, Proc. U.S. Nat. 
Mus., Washington, vol. 37, p. 316. 
M. ovum is larger than Pleurobrachia pileus, becoming 55 mm. long. 
Body egg-shaped in outline, the mouth being at the pointed pole and the 
apical sense-organ at the broad end. The body is strongly compressed 
in the plane perpendicular to the tentacular radii, and thus the creature 
superficially resembles a much-flattened Pleurobrachia. The apical 
sense-organ contains a mass of small concretions and is surrounded bya 
