BRITISH FOSSILS. 



5 



History. — When Brongniart made the first scientific attempt to 

 classify trilobites, his genus Asaphus confessedly stood as debateable 

 land between Calymene and Ogygia^ but he clearly pointed out that 

 the type of the genus, when restricted, should be Asaphus cornigerus, 

 a species possessing smooth eyes and eight joints to the thorax. The 

 characters " Tubercules oculiformes reticules " were drawn from the 

 present species, and the figures also from Dudley specimens. Dalman 

 followed, without much improving this classification, but Quenstedt, in 

 1837, clearly separated the group of trilobites with facetted eyes, 

 (meaning thereby with large prominent facets), and Emmerich named it 

 Phacops. In the treatise on the " Morphology, Classification, &c., of 

 Trilobites," published 1845, in Leonhard and Bronn's " Neues Jahr- 

 buch," Emmerich pointed out the subdivision Dalmannia, making Pli. 

 caudatus (Briinn.), its type. Burmeister, however, has thought fit to 

 retain all under Phacops^ and the characters of one group pass so gra- 

 dually into those of another, that we prefer following his plan, though in 

 the Memoirs of the Geol. Survey, just published, we have adopted the 

 genus (vol. ii., part 1, page 337.) Sir R. I. Murchison, in 1837, gave 

 figures of the ordinary and the tuberculated forms, and there was quite 

 sufficient to justify him in making the var. longicaudatus a species. 

 After observation of many intermediate forms, however, we unite them 

 without much doubt of the propriety of so doing. Green, in 1832, did 

 some service by publishing his casts, and he would have done better had 

 he not permitted them to be carved to make them more plain. Milne 

 Edwards adopted the previously published species in his excellent work 

 on recent Crustacea. Professor Burmeister confused the synonyms of 

 this species, by too hastily uniting P. mucronatus with our var. /S ; Dal- 

 man had well separated the two in 1826. In uniting, as we have done, 

 the var. ^ to the typical form, we believe we shall be justified by most 

 naturalists, when the numerous varieties common in every collection are 

 examined with a view to this. The combination of characters is the 

 same in both, and variation in isolated points should always be sus- 

 pected. The reference of such dififerences to sex is merely speculative, 

 and will require much observation to confirm. 



British Localities and Geological Range. — Distributed throughout 

 the Silurian districts, from the lower Llandeilo flags to upper 

 Ludlow rock. The shales of Kirkcudbright, Scotland (J. W. S.) ; 

 rarely in the Coniston limestone, Westmoreland ; quoted also by Prof. 

 Sedgwick, from Coldwell, ditto (Geological Journal) ; this is a mistake, as 

 is also the quotation in the list of fossils, zZ>., vol. i., p. 20. Upper Silurians 

 of Denbighshire (J.W. S.). Near St. Clairs and Narberth, in Llandeilo 

 flags. Marloes Bay, Pembrokeshire (Phillips). Dynevor Park, and nu- 

 merous localities south and west of Llandeilo ; also Rhiw-rhwych and 



