224 Parallelism of the Palaeozoic Deposits 



atus limestone, and in the Delthyris shales of the State of New 

 York. It has been described by Mr. Green under the name of 

 Asaphus micrurus. According to Mr. Troost, (sixth Report,) it 

 exists also in Tennessee. [The Asaphus pleuroptyx. Green, is 

 also identical with P. Hausmanni. — J. H.] 



P. Dahnani, Portlock. — Discovered by Mr. Portlock in the 

 lower Silurian stage of Ireland, this species occurs in the same 

 horizon in the Trenton limestone of New York.* Closely allied 



labella being 



to the P. Downin 



more triangular and greatly contracted near the base. 



P. limulurus. Green. — The only difference between this spe- 

 cies and the P. mucronatus, Brongn., or P. longicaudattis, Murch., 

 consists in the form of the tail, which is shorter with a wider 

 base. If this difference, observed in a greater number of speci- 

 mens than we have been able to see, obliges us to maintain the 

 specific separation of P. limulurus, perhaps it might be consider- 

 ed as intermediate between the P. mucronatus and P. caiidatus. 

 There is besides, with these two species a great similarity of po- 

 sition. It is found in the schistose slates of Niagara, as the two 

 preceding species are in the slates of Wenlock. The P. niu- 

 cronatitSj nevertheless, in Brittany, descends as far as the lower 

 Silurian schists, while the P. caiidatus remains limited to the 

 Wenlock 



We 



lurus 



gidium separated, too incomplete to be referred with certainty 

 either to the P. limulurus or P. caiidatus. These specimens are 

 not rare in the cliff limestone of Ohio and Indiana, which cor- 

 responds to the Niagara group. One of these incomplete pygi- 

 dia belonging perhaps to the P. limulurus was given to us at 

 Dayton, Ohio, by Mr. Vancleve, as coming from a yellow limestone 

 which appears to be intermediate between the two great Silurian 

 stages. 



P. macropthalmus, Brongn. ; {Calymene bufo, Green; Asa- 

 phus megalopthalmus, Troost; C. latifrons, Bronn.) — This spe- 

 cies in America as in Europe, is one of the most widely spread, 

 principally in the Devonian system. We do not know it person- 

 ally lower; nevertheless we have seen specimens of it coming 

 from Gaspe (New Brunswick), a locality which to judge from 

 other fossils, appears to be upper Silurian, and Mr. Conrad has ci- 

 ted it also in the Delthyris shales, (No. 15.) Nothing is more 

 common, on the contrary, than this trilobite in the Devonian sys- 

 tem as we limit it. It occupies principally the lower parts from 

 the Schoharie grit to the Hamilton group. In the state of New 

 York it is at the deposition of the extensive shales of the Hamil- 



* [This species is described as P. callicephala in the Palaeontology of'N. Y., vol. i, 

 p.247._J. H.] 



