TRILOBITES. 697 
Terminology.] 
are, on the cephalon, known as the cheeks, x; on the thorax and pygidium as the 
pleure, p. The two longitudinal grooves dividing the entire test are the dorsal fur- 
rows. The glabella partakes of a segmentation similar to that of the thorax, but 
incomplete, the dividing grooves rarely extending across it. These grooves are the 
lateral glabellar furrows and are usually in three pairs (sometimes four, and sometimes 
wholly obscured) which are numbered by pairs from the anterior backward (1', 2', 3’). 
Of the lobes formed by these furrows the anterior, or frontal lobe, 1, is large and 
unpaired; thence backward the lateral lobes are numbered to correspond with the 
furrows, each lobe lying behind the furrow with which it corresponds numerically, 
(1, 2, 3). The posterior end of the glabella is limited by a transverse furrow, the 
occipital groove, og, behind which lies a distinct segment or occipital ring, or; both of 
these extend on to the cheeks of the cephalon and form a coalesced segment. 
The lateral expansions of the cephalon or the cheeks are usually divided into two 
parts by a facial suture, s, which extends from the posterior or lateral margin to 
the anterior margin. The test was readily separable along these lines after the 
sloughing of the integument or the decomposition of the lining tissue. The outer 
or separable portions are known as the free cheeks, fc, the inner portion between the 
sutures and the dorsal furrows, as the fixed cheeks, x. In a few genera the facial 
sutures are obscure or not developed, but where they exist the cephalon consists of 
three plates, two free cheeks and a central intrasutural plate to which the term 
cranidium is here applied. The cranidium consists of the glabella, fixed cheeks anda 
greater or less portion of the occipital ring. The outer lateral margin of the cephalon 
may be thickened into a border, b, which meets the occipital ring at the outer posterior 
angle of the cheek (genal angle). This angle may be obtuse, acute or produced into 
spines of greater or less length (genal spines, sp). The eyes, which are present in all 
but a few very early genera, are situated on the cheeks and are traversed by the facial 
sutures which leave the inner portion of the eye-node ( ‘palpebral lobe, n) on the fixed 
cheek, and the outer, visual portion (e), or that bearing the lenses, on the free 
cheek. 
The thorax, T, is composed of a variable number of movable, separable parts or 
segments, sg. The axis of each bears an anterior ring which is overlapped by the 
outer ring of that preceding, and thus forms an articulation. The groove between 
the double axial ring of each segment is called the articulating groove. The pleure 
are frequently beveled on their anterior surface near their extremities, so that 
adjoining segments readily slipped over one another when the animal contracted or 
enrolled itself. The lateral parts of the segments are frequently divided transversely 
by a pleural groove, pg. The extremities of the segment may be acute or obtuse. 
