270 
A. E. Verrill — North American Cephalopoda. 
when more open the aperture is still usually somewhat angular ; the 
anterior sinus is narrow and extends downward and forward. 
The eye-lids form, when nearly expanded, an irregular oval, the 
longest diameter placed transversely and somewhat obliquely, while 
the narrow and deep sinus extends forward and somewhat downward. 
When partly closed (Plate XXIX, fig. 5) the opening between the 
lids generally becomes more oblong and sometimes approaches a 
triangular form. 
The mantle is thick and very muscular ; its anterior margin has a 
concave 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 transverse 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 inter- 
mediate 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 scolloped 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 one 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 fold, commencing near 
the base and running to the tip. Those of the second pair have a 
broader, membranous fold on the lower-outer angle, along the whole 
length. Those of the third pair are stouter than the others, and much 
compressed laterally, with the outer surface rounded, close to the base, 
but becoming compressed and keeled farther out, and having a high 
median ridge along its middle region, becoming narrow toward the 
tip. The ventral arms are trapezoidal in section, with a narrow fold 
