HUDSON-RIVER GROUP. 255 



TRILOBITES OF THE HVDSON-RIVER GROUP. 

 307. 1. TRINUCLEUS CONCENTRICUS. 



Pi,. LXVII. Figs. 1 a- A. Also Pl. LXV. Fig. 4. 



The specimens from the slates of the Hudson-river group are more flattened than those 

 from the Trenton limestone. In a large number of specimens examined, there are four 

 rows of punctures in front of the buckler, while a few have but three rows ; on the other 

 hand, those with four rows are rare in the Trenton limestone, while those with three rows 

 are the prevailing form. This difference, however, cannot be regarded as essential, for in 

 a few instances there are five rows, and very frequently five or six rows on the maxillary 

 margin, near the posterior termination of the buckler. 



The specimens in the Hudson-river group rarely preserve the posterior spine of the 

 glabella, but it is shown in fig, 1 d, and is never wanting except from accident. The long 

 spines from the posterior angles of the buckler correspond in the specimens from the two 

 rocks. 



The thorax consists of six free articulations, the lateral ones being bifurcate or grooved ; 

 the caudal shield has seven segments in each lateral lobe, and about fourteen in the central 

 lobe. The crust is exceedingly thin, almost always separating ; and when preserved in the 

 shaly strata, rarely shows any surface markings. The animal had the power of contracting 

 or folding itself, at least to a certain degree, though this does not appear to have been its 

 habit. 



Fig. I a. A large individual, destitute of spines upon the posterior angles of the buckler and. glabella. 



The border in front of the glabella has three distinct rows of punctures, four in front of 



the cheeks, and five on each side, with six or seven near the base. 

 Fig. I b. The thorax and caudal shield enlarged, to show more clearly the character of the segments. 

 Fig. 1 c. A smaller specimen, preserving the spines of the buckler. There are four rows of punctures 



in front, and six on each side of the buckler. 

 Fig. 1 d. The buckler, preserving the posterior spine of the glabella. 



These specimens are from the soft shales in the upper part of the Hudson-river group, at 



Loraine. 

 Fig. 1 «. Fragments of the cephalic border, showing a variable number of rows of punctures in 



front ; one having five, with seven or eight at the posterior margin. 

 Fig. 1 /. A portion of the marginal fillet, where the crust is partially removed, showing the little studs 



or points which fill these pores from below. 



