396 Prof. F. M‘Coy on the Classification of 
5. Lymnapiapm. Carapace a vertical, bivalve, oblong shell 
inclosing the whole body. Eyes two, semicompound, either 
separate or united in one medial mass. Feet 20 to 80 pair. 
(Type Lymnadia, &c.) 
(Fam. Trilobitade.) 
Homologies of the ‘cephalic shield’ of Trilobites.—This has 
been less attended to than almost any part of their structure. 
The apparently anomalous nature of the facial suture has been 
spoken of by Burmeister, who saw no clue to its nature ; the na- 
ture of the parts outside the eye-line, or ‘ wings’ as they were 
called, has also been alluded to as inexplicable ; while those who, 
comparing the Trilobites with Branchipus, supposed the body of 
the animal to occupy the axal lobe only, have expressed their 
astonishment at the eyes being placed on the lateral lobes, or 
‘cheeks.’ When we bear in mind that the carapace of a crab, 
for instance, is a great backward prolongation of one of the rings of 
the head, and is quite distinct from the posterior cephalic and the 
thoracic segments which it covers, and which exist in a membra- 
nous state beneath it, we are prepared to admit, that though the 
segmental furrows on the glabella of many Trilobites indicate 
cephalic rings, they by no means prove the cephalic shield to be 
formed of the anchylosis of such rmgs, which may only exist be- 
low, impressing it like the various regions on the back of a crab. 
To determine of what rings it is composed, I started with the 
main characteristic of the first ring of all Crustacea, which is, to 
bear the eyes when they are present ; the second and third bear 
the antenne, and the remainder of the cephalic rmgs bear the 
parts of the mouth. The eyes of Trilobites, when they exist, are 
always connected with the piece anterior and external to the eye- 
hne; this piece is usually continuous from side to side at the 
front margin, and I think is probably the first or ophthalmic 
ring ; its lateral portions produced backwards, and bearing its 
peculiar appendages, the eyes, with it: every ring being theoreti- 
cally divisible ito six pieces, affords an explanation of the suture 
which sometimes separates the two parts in front, and even of 
the rostral shield of Calymene (if it belongs to this ring). On 
this view the facial suture becomes at once intelligible as the 
line of separation between the first and second cephalic rings, 
analogous to the divisional line between one thoracic ring and 
another. The piece within and behind the eye-line should on 
this supposition be the second or antennary ring ; and as remark- 
ably supporting this, I must refer to p. 42 of my ‘ Synopsis of 
the Silurian Fossils of Ireland,’ where I announced the discovery 
of the remains of antenne, as a deep pore on each side of the 
