[81] CEPHALOPODS OF NORTHEASTERN COAST OF AMERICA. 291 



ribs; anterior and posterior portions expanded, the latter with the edges 

 involute, and forming a terminal hood or hollow cone. 



OMMASTREPHES D'Orbigny (restricted). 



OTnmastrephcs (pars) D'Orbigny, Voy. Am. Merid., 1835; Cephal. Acetab., p. 341. 

 Illcx and Todarodes Steenstrup, Oversigt K. Danske Yidensk. Selsk. Forbandl., 1880, 

 p. 90. 



Body elongated, pointed posteriorly. Caudal fin broad, transversely 

 rhomboidal. Pen narrowed behind the middle, with a strong median 

 rib and large marginal ribs on each side; near the posterior end thin and 

 concave, expanded into a lanceolate form ; at the tip involute and slightly 

 hooded. Head large. Eyes with lids, having a distinct sinus in front. 



Arms stout, the third pair usually stoutest, with a dorsal keel ; all the 

 arms have marginal membranes, strengthened by transverse muscular 

 ridges, exterior to the suckers. Suckers of the arms deep and oblique, 

 with horny rims, which are strongly denticulate on the outer margin, 

 the median tooth usually largest. Tentacular arms rather long and con- 

 tractile, stout, with a moderately wide terminal club, which has along its 

 middle region two rows of large central suckers, and a row of smaller 

 marginal ones alternating with them on each side; proximal part of 

 club with small denticulated suckers only; distal part of club with four 

 to eight rows of small denticulated suckers. 



Siphon-tube placed in a groove on the under side of tbe head, and 

 attached to the head by a lateral bridle on each side behind the eyes, 

 and by a pair of bridles on its dorsal surface, at the bottom of the 

 depression in which it is lodged. Terminal orifice transversely elliptical, 

 furnished with an internal valve. The depression back of the siphon 

 is smooth in our species, in some other species longitudinally furrowed. 



Mantle-fastenings ("apparatus of resistance"), situated on the basal 

 extension of the siphon, consist of two large triangular bosses, each 

 with an elongated and somewhat ear-shaped longitudinal fosse, and a 

 shallower transverse one. On each side of the inner surface of the 

 mantle is a corresponding T-shaped curtilage, consisting of a short, 

 raised, longitudinal ridge, swollen posteriorly, and a lower transverse 

 ridge, which fit closely into the fosses on the siphon. Tbe dorsal side 

 of the head has a median longitudinal facet, that fits upon its coun- 

 terpart on the mantle, over the anterior part of the pen, which gives it 

 support. 



The nuchal crests are formed by a transverse tegumentary fold 

 behind the eyes, from which run backward, on each side, three longi- 

 .tudinal lamella?, which are delicate, and have a sensory (perhaps olfac- 

 tory) function. 



Buccal membrane seven-angled, thin, corrugated on the, inner surface, 

 destitute of suckers. 



Branchial auricles and gills large. Liver massive, stomach and ccecal 

 appendage volumiuous. 



