CRUSTACEA. 17 



Observations. Specific Diagnosis. The diagnostic characters of this species 

 are very clearly defined, and may be summarized as follows: (a) the axial 

 row of spines extending as far as the pygidium, (l>) the short, stoul spines 

 on the genal angles, (c) the strongly protuberant glabella, (d) the com- 

 parative absence of tuberculations, except on the glabellar surface, (e) the ten 

 or eleven crenulations upon each side of the sub-marginal furrow of the 

 cephalon, (f) the dichotomous annulations of the pygidium. In the summary 

 of these features Phacops cristata differs from any species known outside of the 

 State of New York, and therefore cannot be satisfactorily compared with any 

 extra-limital forms in the United States. Phacops fecundus, Barrande, from 

 the F, G and H etages of Bohemia, and the lower and middle Devonian 

 of the Eifel, Westphalia and the Hartz, presents a striking similarity in 

 outline, in the protuberant glabella and the grooved pygidial annulations. 

 Phacops Logani, Hall, from the Lower Helderberg group, hears a pygidium with 

 similarly bifurcate ribs and obtusely terminated axis, and a cephalon with minute 

 spines on its genal angles, but it is distinguished from Ph. cristata by the con- 

 spicuous nodes along the margins of the axis of the thorax, the stronger tuber- 

 culatum of the surface, and the absence of an axial row of spinules. Phacops 

 Trajanus, Billings, also from the Lower Helderberg group, bears bifurcate annu- 

 lations on the pygidium. hut as this species is incompletely known, its further 

 resemblances to Ph. cristata cannot he indicated. As described and limited in 

 the original description (loc. cit.) Phacops cristata was made to include all the 

 forms of this genus occurring in considerable abundance in the Schoharie grit. 

 and these exclusively, with the exception of a single specimen from the Upper 

 Helderberg limestone at the Indian quarries, Onondaga county. The species 

 Phacops bombifrons was erected at the same time for certain cephalic shields and 

 pygidia from the Upper Helderberg limestones, in which a degree of similarity 

 to Phacops cristata was acknowledged, but in which a more prominent and pro- 

 tuberant glabella, wider occipital furrow, narrower palpebral sulcus and stronger 

 palpebral lobe, were regarded as distinguishing characters. With due allowance 

 made for differences in the mode of preservation in the sandstone and lime- 

 stone of the Upper Helderberg, the specific identity of forms referred to 



