46 Fossils of the Lowe?' Silurian RocJcs of Canada. 



Isotelus gigas is of an oblong oval form, the two extremities bein^ 

 about equal. The middle portion, or the thorax, as it is called, consists of 

 eight articulations or segments, which at their ends are slightly curved for- 

 ward and flattened to a thin edge upon their anterior side. The tail, the 

 lower portion of Fig 9, is smooth elevated in the centre, and gradually de- 

 clining to the margin all round. This part of a trilobite is called the pygi- 

 dium by pateoutologists, and is, in most species, furrowed with grooves in 

 such a manner as to render it somewhat difficult to determine where the line 

 between it and the thorax should be drawn. In tliis species it is so distinct 

 that no such question can arise. The head is composed above of three 

 pieces, the two outer portions called cheeks, and the central the glabella. — 

 The latter is but slightly convex in this species, but in others, it is elevated 

 and variously lobed. The sutures or lines of division between the cheeks 

 and the glabella, stai't from the middle of each of the side lobes of the body 

 and curve inwards to the lower corner of the eye, then form a short semicir- 

 cle half round that organ and thence proceed with an outward curve to the 

 centre of the front part of the head. The eyes are prominent in perfect 

 specimens, and in the shape of a crescent with the angles rounded. The 

 greater number of the species of this race are strongly trilobed by two deep^ 

 nearly parallel furrows, which extend from the head to the extremity of the 

 tail. In /. gigas only the thorax is much trilobed — the furrows being but 

 obscurely visible on either the anterior or posterior extremities. 



Fig 11 represents a part of / gigas, called 

 the hyposioma, an organ which appears to have 

 been analogous to the lahrum. or upper lip of the 

 insects of the present day. The hypostoma is 

 often found separated from the other portions of 

 the trilobites. The one figiu-ed, belonged to an /. 

 gigas of medium size. Much larger specimens 

 are occasionaly met with, but generally they are 

 smaller. 

 Fig 11. Hypostoma of Isotelus Gigas. 



In the Trenton Limestone fragments of this great trilobite have the 

 appearance of smooth or slightly punctured pieces of black shell. The head 

 and tail are the parts most frequently found perfect, and are easily recog- 

 nized ; but good specimens with all the parts in their natural connection 

 are exceedingly rare. It is said that they have been seen eighteen inches in 

 length, but from four to eight inches appears to be the prevailing size in our 

 Canadian rocks. Isotelus gigas commenced its existence about the period 

 of the Black River Limestone, and disappeared from the seas at the close of 

 the Trenton limestone epoch. The generic name is from the Greek, " isos,'* 

 equal ; and '' telos," end ; in allusion to the equal extremities of the animal^ 

 " gigas," a giant. 



Calymene senaria. — This fossil is very distinctly divided, from one 

 end to the other, into three lobes, and thus presents in full perfection tha 

 characteristic feature which gave name to the race. The specimens are of 



