CRUSTACEA. 25 



A still more intimate agreement may be found in the form Phacops laHfrons, 

 var. occitanus, Tromelin and Grasset, as described by Barrois (Ann. de la Soc 

 Geol. du Nord, vol. xiii, p. 75, L885), from the lower middle Devonian of the 

 Herault. This variety, with all the essential specific characters of Ph. latifn 

 has the eyes less elevated, the basal glabellar annulation smaller and without' 

 the strong, central tubercle; the tuberculatum of the glabella is liner, and 

 the granules on the pleurae of the pygidium less strongly marked. These 

 different forms undoubtedly represenl the same species in trans-oceanic faunas, 

 and are only such variations as might be expected in descendants of the same 

 ancestors subjected to different conditions of life. With Phacops fecund us, Bar- 

 rande, of the later formations (Hercynian) of the Bohemian Basin, etages F, 

 G and H, there is also a close agreement except in the occurrence of silicate 

 annulations upon the pygidial pleura? in the latter species. 



The specimens from the Eureka District and Lone Mountain, Nevada, identi- 

 fied by Walcott (foe. at.) as Phacops rana, Green, present points both of similarity 

 and difference with the typical forms of the species, at the same time showing 

 distinct features allying them to the Upper Helderberg species Phacops cristata. 

 They resemble the former species in (a) the absence of spines at the genal 

 angles, (b) the apparently simple pleural annulations of the pygidium; they 

 are similar to the Upper Helderberg form in (a) the inconspicuous palpebrum, 

 (b) the strong palpebral lobe, (c) the deep and broad occipital furrow which 

 bears no trace of the transverse ring behind the frontal lobe, (d) the elevated 

 and narrow occipital ring, (e) the hypostoma with strong antero-lateral eleva- 

 tions enclosing shallow depressions, (f) the obtuse and flattened termination of 

 the axis of the pygidium, (g) the interrupted terminal annulations of the axis. 

 This western form, as observed by Mv. Walcott, thus appears to be a variety 

 connecting the two species named and may eventually be regarded as of distind 

 specific value. 



Distribution. Upper Helderberg group. The occurrence of Phacops rana in 

 this formation is not yet establi>hed beyond question A single specimen is 

 labelled from the Corniferous limestone near LeRoy, Genesee county. Hamil- 

 ton group : In the Marcellus shales rarely, at Chapinville, Ontario county : in 



