NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 107 



We are in possession of over a dozen cephalic shields of this species, and they all agree 

 to a remarkable extent. 



This fine species is in no danger of being confounded with any other American form. 

 It has close analogues, however, in A. vcsiculosa Bey rich (Barrande, Silur. Syst. p. 715, PL 

 38, figs. 13-21), and A. Verneuilli, (Id. PI. 38, figs. 1-9). The occipital and genal spines, 

 nevertheless, are longer, the ocular peduncle is more elevated and more cylindrical, (in this 

 resemblin^ A. mira.) the lateral lobes are more confluent with the cheeks ; the movable 

 cheeks are more depressed, and the whole head is transversely more elongate. 



Besides the foregoing species in our collection from this interesting locality, the following 

 described species have not been met with by us. 



Bucania chicagoensis McChesney, New Paleozoic Fossils, p. 69. 



" crassolaris " " " " p. 91. 



" -pervohita " " " " p. 91. 



Orthoceras cameolare " " " '' p. 93. 



" stricelinealum " " " " p. 94. 



Besides the organic remains already enumerated, we are in possession of a few others 

 in a condition too imperfect for determination ; among which may be mentioned specimens 

 resembling StropJiomena and Naticopsis a cluster of eight or ten small, hollow conical, slight- 

 ly bent tubes, having the appearance of a small Dentalium ; also a considerable mass of 

 small stellate and acicular crystal-like bodies, resembling freshly fallen snow-flakes, which 

 may have had an organic origin. 



DECEMBER 22, 1864. 



SUPPLEMENTARY NOTE. 



About a month after the foregoing paper had been accepted for publication, a pamphlet 

 appeared, from the pen of Professor James Hall, entitled, " Account of some netv or little knoivn 

 Species of Fossils from the Niagara Group." This pamphlet, while mainly devoted to fossils 

 from Wisconsin, embraces a notice of twenty-two species of fossils from Illinois, most of 

 which are referred to Bridgeport, and the remainder of which, as I have since learned from 

 examinations kindly permitted by Prof. Hall, come also from that locality. Of these, only 

 eleven species are quoted from Bridgeport} 



We had been aware, early in December, 1864, that Prof. Hall was at work upon fossils 

 from the Niagara group of Wisconsin ; and one of us also informed him that we had just 

 completed a monograph of the fossils of Bridgeport, a work which had been in progress 

 for about two years. It did not occur to either of us that Prof. Hall's plan, as we under- 

 stood it, would embrace fossils recognized only at the locality upon which we had been 

 especially engaged. From this misunderstanding has resulted a little synonymy and a little 

 clashing of identifications. A brief review of Prof. Hall's paper seems, consequently, to be 

 called for. 



1. The following twelve species are quoted by Prof. Hall from Bridgeport, and other local- 

 ities. 



Jllcenus insignis, I. armatus, Sphcerexochus minis Beyrich ; Amboni/chia Aphcea, Avicula undata, 

 Avicula emacerata, Conrad ; Cypricardinia arata, Modiolopsis Dictceus, Modiolopsis rectus, Modiobp- 

 sis subalatus, Amphiccelia Leidyi, Subulites ventricosus. 



1 The locality of Plerinea Brisa Hall, is not given, but it is now known to be from Bridgeport. 



