272 A BETIEW OF THE TRILOBITES. 



is still surrounded. In the present sketch, it is proposed to consider 

 our subject under the following heads : — The general organization 

 of the Trilobites ; their probable habits and affinities ; their geologi- 

 cal relations ; and their classification. 



1. General characters. Viewed both transversely and longitudi- 

 nally, the trilobite presents a tripartite form. Transversely, we have 

 the head, the body or thorax, and the abdomen, or so-called tail. 

 Longitudinally, the form becomes three-lobed, by the presence of two 

 linear depressions or furrows, extending in general, almost from one 

 extremity of the animal to the other. Occasionally, however, as in 

 the genus Homalonotus for example, these longitudinal furrows are 

 but very slighty developed. 



We will consider separately the structural characters of the head, 

 the thorax, and the abdomen : or rather of the crustaceous integu- 

 ments by which the back of these parts was defended ; for of the in- 

 ternal conformation of the trilobites, our knowledge is almost en- 

 tirely conjectural. 



2. The anterior portion of the trilobite is covered by a single shield 

 composed of several united pieces. To this shield the term of luck- 

 ier or head-shield is commonly applied. The middle division, formed 

 by the anterior prolongation of the two longitudinal furrows men- 

 tioned above, is called the glabella. In some species the glabella is 

 very strongly pronounced, whilst in others it is scarcely raised above 

 the general level of the head-shield. It is usually lobed or furrowed 

 by short transverse grooves, or variously embossed ; although occa- 

 sionally quite smooth and simple. In some species again, it narrows 

 towards the summit, whilst in others it expands. The separate pieces 

 of which the head-shield is composed, are united by distinct su- 

 tures : a character peculiar, amongst crustaceans, to the trilobites. 

 This, as suggested by Baebande, probably facilitated the periodic 

 casting of the shell. One of these sutural lines, the great or facial 

 suture, is of considerable importance as a classification element. In 

 the majority of instances, it passes on each side of the head-shield, 

 from the angles {calif mene), or from the lateral or lower border (pha- 

 cops, asaphus) along the inner margin of the compound eyes (where 

 these exist), and either surrounds the glabella, or terminates beyond 

 the anterior margin of the shield. In the latter case, it is said to be 

 open above. The buckler or head-shield is thus divided into three 

 pieces ; the middle piece, including the glabella and the " fixed cheeks" 

 or parts between the glabella and facial suture ; and the side pieces, 

 or ordinary cheeks. The latter are very commonly wanting in trilo' 

 bite specimens, or are found separated from the other portions of the 



