82 AETICULATA. 



defined. The eyes ai'e sessile and compound. The head is cov- 

 ered by a crescent shaped shield or hucMer i the thorax consists 

 of from two to twenty-six segments ; and the j^^/^^'^^m^tt?., or ab- 

 dominal shield, is composed of a number of joints more or less 

 anchylosed. The buckler is divided b}' a longitudinal depi'ession 

 into cheeks, or lateral areas, and the glabella, or middle area. A 

 suture running from the anterior and posterior sides of the eye 

 outward is called i\\e facial suture / a prominent accessory piece 

 on the under surface of the head, covering the mouth, is the 

 hypostoma. The union of the head-crust by sutures is a remark- 

 able peculiarity of Trilobites, — not being traceable in any other 

 Crustaceans, and only found in true Insects of the present time. 

 The genera of Trilobites are distinguished mainlj' by the form 

 and markings of the head and tail, and by the eyes. Variations 

 in the length of the cephalic and caudal spines, and in the promi- 

 nence of the head-lobes, have been considered indications of dif- 

 ference of sex, No antennae have been detected. Thoracic 

 jointed limbs and branchiae have recently been discovered, and 

 the abdomen probably bore membranous respiratory swimmerets. 

 They probably lived gregariously in the shallow waters of bays 

 and coasts, and from the form of the labrum mouth, it is inferred 

 that they were carnivorous. The embryos are simple discs. 

 Barrande has made out the probable eggs. More than 500 species 

 of Trilobites are known, grouped in about TO genera. The order 

 appeared in the Cambrian, culminated in the Upper Silurian, and 

 disappeared in the Carboniferous. The spinous feature reached 

 its maximum in the Devonian. Trilobites, incapable of rolling 

 themselves up, are of more ancient geological date than those 

 which can ; they are chiefly found in the lower Silurian. Those 

 having the power of rolling themselves up, have larger and more 

 projecting eyes, a tougher horny membrane, and a caudal shield 

 resembling the buckler. 



No. 142. [376, Cast]. Asaphas megistOS, Locke. 



This specimen shows well the loug posterior processes of the cephalic shield, 

 which in tliis specimen extends nearly to the end of the tail. It is from the 

 Cincinnati group (Lower Silurian), Cincinnati, Ohio. 



No. 143. Asaphas gigas, Dal man. 



Pygidium. From Trenton Limestone, Trenton Falls, N. Y. 



