Crustacea. | LOWER PALZOZOIC ARTICULATA. 139 
is added a peculiar marginal transverse “ rostral suture,” dividing the head into four pieces. These sutures exist 
in no recent Crustacea, being only now known in true insects. By some recent continental Paleontologists all 
outside and anterior to the eye-line is called the ‘ cheeks,” and all within or posterior to it is called the “ glabella ;” 
by a greater number of writers however (and in the following descriptions) the latter term is used only for the 
elevated middle portion of the head, and the triangular depressed space on each side form the cheeks, through 
the middle of which the “‘ eye-dine” runs; the glabella is divided by not more than three lateral sulci into never 
more than four lobes; at the base of the glabella a strong furrow going across is called the “ neck furrow,” 
behind which is the “xeck segment,” or spira; these sulci indicate the attachment of the buccal muscles. 
The eyes are reniform, prominent, situated in a gap in the eye-line, beneath and external to the eye-lid; they 
have all the “ semi-compound eyes,” the outer thin layer of the cornea being entire and smooth, the inner thick 
layer facetted and perforated; Burmeister notes that the larger the eye of living crustacea, the thinner the 
cornea, hence why the large-eyed Phacops having a thin easily-lost outer cornea, seems to have a facetted eye. 
I have drawn attention to a deep puncture on each side of the antennary lobe, in the furrow separating the 
glabella from the cheeks near the front margin, which I have called the “‘antennary puncta,” and which I 
conceive gaye passage to small antennze like those of Branchipus, of which organs they hold the normal 
position. Attached to the under side of the anterior edge of the head is an ovate crustaceous piece varying 
in shape according to the genus, its anterior part is dilated into two horn-like processes following the curve 
of the anterior margin of the shield, the surface of this organ is marked with fine irregular impressed strize like 
the rest of the under surface of the crust of Trilobites; it is believed to be immovable, and corresponds 
to the enlargement in front of the mouth in some of the recent Phyllopoda, and is, as in them, named 
the “ Hypostoma.” The Thorax is formed of a variable number of segments (constant for each genus), 
forming the movable portion of the crust between the cephalic and caudal shields; it is in general strongly 
trilobed longitudinally, the middle and most convex portion termed the “axis,” and the lateral, lobes or 
pleurze; each segment has the axis divided in many genera into a raised, posterior exposed portion, and 
a depressed anterior articular portion, which is concealed under the preceding segment when the animal 
is extended, becoming visible as it rolls up, or on the fracture of the edge of the preceding ring; the 
pleure are straight, or bent down at the end; in the latter case the end often ‘sharpened at its anterior 
edge for contraction, or provided with a triangular ‘facet ;’ one of its sides parallel with the anterior edge, 
its base at the distal extremity, and its apex reaching about one-third towards the axis, its thickened posterior 
margin joining the anterior margin of the segment at an obtuse angle, at the point where the end of the 
pleura begins to bend down, has, from its form, been called the “knee” by Pander, Portlock, &c.; the 
same point has by others been termed the “fulcrum.” In most cases a deep groove extends transversely 
from the axis along the middle of each pleura as far as the ‘knee,’ and then its extremity is often bent 
backwards at an obtuse angle, I propose to term this the “ plewral groove,” and Dr Burmeister suggests, with 
great probability, that the membraneous gill-feet were attached to the corresponding ridge on the under side ; 
caudal shield or “‘ pygidium”™ semi-elliptical of one solid piece, axis and side lobes generally with transverse 
false segmental furrows, rarely corresponding in number on the axis and sides, and not admitting of inde- 
pendent motion. 
Dr Burmeister comparing trilobites with Branchipus, suggests that the narrow body was lodged exclusively 
in the narrow prominent middle lobe or axis extending from the front of the glabella to the end of the 
pygidium, and that the lateral lobes were merely to protect the delicate respiratory feet hanging beneath 
them; these being mere lateral extensions of the true or axal rings renders it impossible they could have 
acted as oars, as has been suggested. In such cases, as Paradowides, where the thoracic segments are 
very numerous, and the abdominal shield disproportionally small, a doubt occurs as to whether some of the 
abdominal segments may not be erroneously counted as thoracic; but as this could only be determined by 
observing the position of the female genital openings, which mark the last thoracic ring, Palzontologists 
are content to call all the free movable segments thoracic, and refer to the abdomen only those which are 
anchylosed into a caudal shield or pygidium. The lateral portions of the body are covered with a thin, tough, 
membrane on the under side, marked with oblique dichotomous impressed strize. 
The homologies of the cephalic shield of Trilobites have been less attended to than almost any part 
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