124 Monograph of the Crustacea of the Cincinnati Group. 



with a slight outward curve to the eye, over which it makes an abrupt 

 outwarrd curve to define a small semi-circular lobe, improperly called 

 the "eye lid ;" its direction from the lower end of the eye varies ac- 

 cording to the genus, and forms a very valuable character ; in 

 some cases the eye lines do not join in front of the glabella, but cut 

 the anterior margin separately, thus dividing the shield into three 

 pieces, and in Calymene, to this latter disposition is added a peculiar mar- 

 ginal transverse "rostral suture," dividing the head into four pieces. 

 These sutures exist in no recent Crustacea, being now only known in 

 true insects. 



By some paleontologists all outside and anterior to the eye line is called 

 the "c/ieefe," and all Avithin or posterior to it is called the "glabella;" 

 by a greater number of writers, however, the latter term is used only 

 for the elevated middle portion of the head, and the triangular de- 

 pressed space on each side form the cheeks, through the middle of which 

 the eye line 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 "nech furrow, behind which is 

 the "neck 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 en- 

 tire and smooth, the inner thick layer facetted and perforated. Bur- 

 meister 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 (McCoy) have 

 drawn attention to a deej) puncture on each side of the anten- 

 nary lobe, in the furrow separating the glabella from the cheeks near 

 the front margin, w^iich I have called the "antennary puncta," and 

 which, I conceive, gave passage to small antennte, like those of Bran- 

 chipus, 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 di- 

 lated 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 strite, 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 re(3ent 

 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 



