TRENTON LIMESTONE. 229 
TRILOBITES OF THE TRENTON LIMESTONE. 
Piates LX. (in part), LXI. - LXV., & LXVLI. (in part). 
40. 2. ILLAZNUS CRASSICAUDA. 
Pu. LX. Figs. 4 a, 0, ¢, d. 
Entomostracites crassicauda. .WAHLENBERG, Noy. Act. R. Soc. Sc. Upsal, 1821, Vol. viii, pag. 27, no. 2, 
tab. 2, fig. 5 & 6; and pag. 294, no. 1, tab. 8, fig. 5 & 6. 
Illenus crassicauda. Dauman, Vet. Acad. Handlingar, 1826, pag. 248, pl. 5, fig. 1 a, b,c. 
— — Histncer, Leth. Suecica, 1837, pag. 16, tab. 3, fig. 4. 
— — Porriocx, Geol. Rep. Londonderry, 1843, pag. 300, pl. 10, figs. 3 a, 3 b, 4, 5. 
Illenus trentonensis. Emmons, Geol. Report, 1842, pag. 390, fig. 3. 
Compare Illenus perovalis, Murcuison, Sil. System, 1839, pag. 661, pl. 23, fig. 7 a, b. 
Oval with the longer axis more or less extended, convex ; buckler large, convex, rotund ; 
posterior extremities obscure ; eyes prominent, distant from the axis; maxillary shield 
small ; thorax distinctly three-lobed, the divisions continuing a short distance into both the 
cephalic and caudal shields ; articulations ten, smooth, slender, those of the lateral lobes 
extended ; caudal shield large, semicircular, convex, having the rudiment of a central 
lobe ; entire surface smooth, or with fine curving- subimbricating lamellose striz. 
The general form of this species is variable from its contractility, none of the individuals 
yet seen being quite perfect. It is distinguished from IsorrLus by the number of articula- 
tions, and by the more rotund and less equal extremities. 
In the rocks of New-York, the cephalic and caudal shields of this fossil are frequently 
found ; but from the delicacy of the articulations of the thorax, they are rarely preserved. 
Fig. 4 a. The caudal shield of a large individual. 
Fig. 4 6. Middle lobe of the cephalic shield, the lateral portions being separated at the sutures, 
Fig. 4 c, Lateral view of a small entire specimen. 
Fig. 4 d. Dorsal view of the specimen, showing the great width of the middle lobe, and abrupt incurving 
of the cephalic shield. 
Position and locality. In the Trenton limestone at Middleville, Watertown, Turin (New- 
York), Carlisle (Pennsylvania), and in the upper part of the Birdseye limestone on Ile la 
Motte ( Vermont). 
