No. 136.] 4:1 



cleus concentricus, a little trilobite wliich has a semicircular 

 buckler marked with three bold prominences or swellings, and a 

 large number of dots or depressions around the border, with a 

 spine on each corner ; its body being less in size than the buckler 

 itself (Fig. 10, No. 1). 



In Fig. 6 is a woodcut (No. 1) of the large Isotelus gigas. In. 

 Fig. 1 are illustrations of several forms : No. 1 is a very rare 

 one, IllcBnus tYentonensis ; No. 2, a very common species, Calymene 

 senaria; No. 3, a small and rare species, Illcznus latidorsata; No. 

 4, a small living crustacean from the Antarctic seas at Cape 

 Horn, introduced as somewhat similar to the trilobites ; Nos. 5 

 and 6, parts of the heads of two rare species, No. 5 having lost 

 all but the central part, and No. 6 having lost the margins or 

 "cheeks;" No. 7 is the head only of the little Trinucleus^ com- 

 mon in many localities, (shown in a perfect condition in Fig. 10.) 



This rock also contains many species of Orthocerata, which 

 in some places almost cover the worn surfaces of the strata ; and 

 a very handsome coiled chambered shell about two inches across, 

 the Trocholites ammonius (Fig. 8, No. 1). There are also several 

 species of Bellerophon, a small coiled shell with a flaring or 

 trumpet-shaped mouth; one of which has the mouth curved or 

 deeply indented at the middle, dividing the edge of the shell in 

 two projections, whence it is called the Bellerophon hilohatus. It 

 is one of the commonest and most characteristic fossils of the 

 rock. We find in it also other varieties of coiled shells of other 

 genera : Murchisonia, coiled in a spiral form ; and Pleuroto- 

 MARiA, coiled more flatly, like a snail-shell (Fig. 8, No. 2). A 

 very large number of small bivalves also occur, the most abun- 

 dant of which are different kinds of Brachiopoda, the general 

 nature of which class was explained in speaking of the Chazy 

 limestone. They are of various genera : Lept^ena, Atrypa, 

 Rhynchonella, Orthis, Strophomena, etc. etc. {Leptcena deltoidea 

 is shown in Fig. 6, No. 3). 



The Trenton limestone also contains several Crinoids, which 

 are exceedingly rare in a state approaching perfection, though 

 short pieces of their stems are common, and their separate discs 

 most abundant. The " Crinoids " are animated forms of a very 

 remarkable character, organized in many respects like an Echinus 

 or " Sea-egg," or like a Star-fish ; but generally bearing clusters 

 of arms or jointed appendages at the summit, and attached at 

 the base by a long jointed column or stem to the ground. This 



