311 



Found in the "shales of the Hamilton group, at Eighteen 

 Mile Creek, N. Y." (Uh-ich, type.) 



Order Trilobit.k. Burmeister. 



The trilobites are extinct Crustacea, wholly confined to the Palaeozoic 

 sea. The body was covered with a carapace, longitudinally divisible 

 into three parts. The anterior portion comprises the head-shield or 

 cephalon, which is usually semi-circular, with a straight posterior border. 

 The central of the three cephalic lobes is the glabella, which is the most 

 prominent part of the cephalon. It is of varying outline, and more or 

 less divided by transverse furrows or pairs of furrows. The last furrow 

 is the occipital furrow, and delimits the occipital ring, which is just 

 anterior to the first segment of the thorax. On either side of the glabella 

 is a pair of cheeks, divided by the facial suture into fixed cheeks (those next 

 to the glabella) and free cheeks (the outermost portion). The latter are 

 often prolonged into genal 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 por- 

 tion 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 back, making the doublure, which, continued backwards, often 

 produces hollow or solid genal spines. To the anterior lower portion of 

 the doublure is attached the lower lip, or hypostoma, which is often found 

 separate. 



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

 lated with each other, and commonly permitting enrollment. They con- 

 sist of a central annulus and lateral pleura). 



The tail, or pygidium, consists of a single piece, comprising a central 

 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 appendages, 

 which included antennae, mouth parts, and legs, comparable in a general 

 way to those of modern Crustacea. 



Note. — For a fuller account, and for a bibliography, the student is re- 

 ferred to Zittel's Text-book of Palaeontology. Of special interest are the 

 recent papers by Walcott, Beecher, Matthew, and others, on the append- 

 ages and development of Trilobites, published in the American Journal of 

 Science, the American Geologist, and other periodicals. 



