TRENTON LIMESTONE. 



229 



TRILOBITES OF THE TRENTON LIMESTONE. 



Plates LX. (in part), LXI. - LXV., & LXVI. (in part). 

 40. 2. ILL^NUS CRASSICAUDA. 



Pl. LX. Figs. 4 a, b, c, d. 



Entomostracites crasgicauda. Wahi.£nberg, Nov. Act. R. Soc. Sc. Upsal, 1821, Vol. viii, pag. 27, no. 2, 



tab. 2, fig. 5 & 0; and pag. 294, no. 1, tab. 8, fig. 5 & 6. 

 nianua craasicauda. Dalman, Vet. Acad. Handlingar, 1826, pag. 248, pi. 5, fig. 1 a, b, c. 



— — HisiNGEB, Leth. Suecica, 1837, pag. 16, tab. 3, fig. 4. 



— — PoRTLOCK, Geol. Rep. Londonderry, 1843, pag. 300, pi. 10, figs. 3 a, 3 J, 4, 5. 

 IlUenut trtntonemu. Emmons, Geol. Report, 1842, pag. 390, fig. 3. 



Compare JlUenui permalit, Murchison, Sil. System, 1839, pag. 661, pi. 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 striae. 



The general form of this species is variable from its contractility, none of the individuals 

 yet seen being quite perfect. It is distinguished from Isotelus 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 b. Middle lobe of the cephalic shield, the lateral portions being separated at the sutures. 

 Fig. 4 e. 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 He la 

 Motte (Vermont). 



