NIAGARA FALLS AND VICINITY 221 



gcnal spines. The compound eyes are situated on the free cheeks, 

 and they are overshadowed by more or less prominent eyelids or 

 palpebral lobes, which are lateral lobes from the fixed cheeks. The 

 facial suture thus passes between the eyes and the palpebral lobes, 

 and when, as is often the case, the free cheeks become separated 

 after the death of the animal, only the palpebral lobes remain on the 

 central portion of the cephalon. The border of the cephalon is 

 often distinctly marked, and is spoken of as the cephalic limb. At the 

 margin it is folded down and under, making the doublure, which, 

 continued backward, often produces hollow or solid genal spines. 

 Near the anterior lower portion of the doublure lies the lower lip, 

 or liypostoma, which is usually found separate. 



The thorax consists of a varying number of segments or rings, 

 articulated with each other, and commonly permitting enrolment. 

 Each consists of a central annulus and lateral pleurae. 



The tail, or pygidium, consists of a single piece, comprising a cen- 

 tral axis and lateral lobes. The axis and the lobes commonly show 

 transverse furrows, corresponding to the divisions of the thorax, and 

 they are often so strongly marked that a line of division between 

 thorax and pygidium is difficult to determine. 



Great advances have recently been made in our knowledge of the 

 ventral side of trilobites. Probably all of them had jointed appen- 

 dages, which included antennae, mouth parts and legs, comparable 

 in a general way to those of modern Crustacea. 



Genus homalonotus Koenig 



[Ety. : 6p.a/.6$, on the same level; vcutos, back] 



(1825. hones foss. sectiles, p. 4) 



Body usually large, depressed above, with abruptly sloping sides. 

 The axial furrows are indistinct or obsolete. Cephalon depressed 

 convex, wider than long, with rounded genal angles, and somewhat 

 produced anterior margin; glabella almost rectangular, smooth, or 

 with faint lateral furrows. Small eyes situated behind the middle, 

 and converging facial sutures are characteristic. Thorax of 13 

 deeply grooved segments. Pygidium smaller than the cephalon, 

 elongate triangular, rounded or produced posteriorly; axis with 10 

 to 14 annulations; lateral lobes smooth or with posteriorly sloping 

 ribs. 



Homalonotus delphinocephalus (Green) (Fig. 153) (Hall. 1852. 

 Pal. X. Y. 2 1309, pi. 68) 



Distinguishing characters. Subtriangular cephalon; subquadrate 

 glabella, widening a little posteriorly; small lateral eyes; acute anter- 

 ior termination of cephalon; non-trilobate character of thorax, nar- 

 rowing rapidly toward the posterior end ; abruptly triangular pygid- 



