ON THE STOCKDALE SHALES. 721 



We have compared the forms discovered in the Stockdale Shales 

 with the figures of the Russian specimens, and with actual examples 

 collected by one of us from the Llandovery rocks of Christiania, and 

 they agree in every particular. 



The species has frequently been confused with Phacops Stohesii, 

 Milne-Edw., a form common in the Wenlock rocks. JFrom this the 

 present species differs in the shape and smoothness of the glabella, 

 the possession of smaller eyes, and the convexity of the axis of the 

 tail. 



The incurved lower margin of the cheek possesses six cavities for 

 the reception of the ends of the pleurae (PI. XVI. fig. 3 h). The 

 interspaces between these increase in size anteriorly, as the posterior 

 pleurae, the ends of which are received by these, do not overlap one 

 another so much as the anterior pleurae, when the animal is rolled up. 

 Indications of similar cavities are seen in a specimen of Phacops 

 JStokesii from the Wenlock Limestone of Dudley, preserved in the 

 Woodwardian Museum, and numbered g|^. 



Roy. Sf Log. Heads and tails of this species are common in the 

 Phacojps-glahei' and Ampycc-aloniensis zones of Skelgill, the latter 

 zone of Browgill, the Acidaspis-erinaceus zone of Torver Eeck and 

 Spengill, and the calcareous bed of Austwick Beck. It is a common 

 Llandovery fossil in many parts of Britain and the continent. 



Phacops (proper) elegans, var. nov. glaeee. (PI. XVI. fig. 4.) 



Tail ^ inch broad, J inch long. Axis one fifth the width of the 

 whole tail, tapering gradually, and extending about two thirds the 

 length of the tail ; marked with two very deep prominent furrows 

 anteriorly, and four or five obscure ones posteriorly. Limb marked 

 by two slight furrows on either side, the under surface exhibiting 

 a thick recurved margin. 



These broad tails are very abundant in one horizon of the Skelgill 

 Beds, and less common in others. They are easily distinguishable, 

 by their great breadth and extreme smoothness, from the tails of the 

 typical P. elegans ; but one or two intermediate forms would indicate 

 that the present Trilobite is but a variety of the normal form. 



A single head, apparently belonging to this variety, and differing 

 from the ordinary form in the size of the eye and some details of 

 the shape of the glabella, is too imperfect to figure. 



The specimens indicate a Trilobite which, when complete, must 

 have measured an inch and a half in length, thus greatly exceeding 

 that of Phacops elegans proper. 



Hor, 6f Loc. Common in the glaher-zone of Skelgill ; rare in the 

 aJoniensis-zone of Skelgill and BrowgiU. 



Phacops (Dalmannites) mtjceoi^atijs, Brongn. (PI. XVI. figs. 5, 6.) 

 Entomostradtes caudatus, Wahl.Nov. Act. Soc. Upsal.vol. viii.p. 25. 

 Asaphus mucronatus, Brongn. Crust. Foss. p. 24. 

 Asaphus mucronatus. Dalm. Vet. Akad. Handl. 1826. 

 Asaphus mucronatus, Hisinger, Lethsea Suecica, p. 13. 

 Phacops mucronata^ Ang. Pal. Scand. p. 10. 

 Phacops mucronata, Emmr. Neues Jahrb. 1845. 



