[ 85 ] CEPHALOPODS OF NORTHEASTERN COAST OF AMERICA. 
diameter placed transversely and somewhat obliquely, while the narrow 
and deep sinus extends forward and somewhat downward. When partly 
closed (Plate XIX, fig. 4) the opening between the lids generally be¬ 
comes more oblong, and sometimes approaches a triangular form. 
The mantle is thick and very muscular ; its anterior margin has a con¬ 
cave outline beneath, forming a slightly prominent angle on each side; 
from these angles it advances somewhat to the slight median dorsal 
angle, which projects forward but little, and does not form a distinct 
lobe, and sometimes it is hardly noticeable, even as an angle, the trans¬ 
verse outline of the edge on the dorsal side being, in that case, nearly 
straight, or advancing a very little in the middle. 
The sessile arms are rather stout, tapering to acute tips. The dorsal 
arms are a little smaller and shorter than the others ; the second and 
third pairs are nearly equal in size and length, the second often a trifle 
the longer; those of the fourth pair are usually intermediate in length 
between the first and second pairs. 
All the sessile arms are stout and armed with similar suckers. Along 
their inner angles, outside the suckers, they are all similarly provided 
with marginal membranes, which rise to about the same height as the 
suckers, on each side. Just proximal to each sucker, on the inner face 
of the arm, arises a thickened, transverse, muscular fold, that extends to 
the edge of the lateral membrane, which often recedes between their 
extremities, so as to have a scalloped outline. 
The dorsal arms are a little shorter and decidedly smaller than the 
others. The two lateral pairs of arms are stoutest and longest, and 
nearly equal, sometimes ope pair and sometimes the other being longest. 
The ventral arms are a little longer than the dorsal and shorter than 
the lateral ones. The dorsal and upper lateral arms are trapezoidal in 
section, with the inner face rather broad. The dorsal arms have a 
slightly elevated, median dorsal crest, commencing near the base and 
running to the tip. Those of the second pair have a broader, mem¬ 
branous fold on the lower outer angle, along the whole length. Those 
of the third pair are stouter than the others, and much compressed lat¬ 
erally, with the outer surface rounded, close to the base, but becoming 
compressed and keeled farther out, and having a high median ridge 
or crest along its middle region, becoming narrow toward the tip. The 
ventral arms are trapezoidal in section, with a narrow fold along the 
outer angle, which is acute, while the inner ventral angle is rounded. 
The tentacular arms (Plate XVIII, figs. 1 a , 2) are long; when extended, 
in fresh specimens, they reach back beyond the base of the caudal fin. 
They are rather stout, rounded-trapezoidal along the peduncular portion; 
along the upper-outer angle a thin fold runs from the base to the tip, 
becoming on the back side of the club a wide carina, which often folds 
down obliquely toward the upper margin of the club; two less marked 
folds run along the inner angles, defining a narrow inner face along the 
whole length, but on this face there are no suckers, except close to where 
