396 Prof. F. M'Coy on the Classification of 



5. LYMNADiADiE. Carapace a vertical, bivalve, oblong shell 

 inclosing the whole body. Eyes two, semicompound, either 

 separate or united in one medial mass. Feet 20 to 30 pair. 

 (Type Lymnadiaj &c.) 



(Fam. Trilobitadce.) 



Homologies of the f cephalic shield 3 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 rings, 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 antennae, and the remainder of the cephalic rings 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- 

 line ; 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 into 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 antennae, as a deep pore on each side of the 



