4 



BRITISH FOSSILS. 



StAUROCEPHALUS ? UNICUS, 



Diagnosis. S. H uncialisy oblongus, sparse granulosus^ glabella gibba 

 eminentissima, corpore piano, cauda expansa transversa. Caput latum, 

 glabella clavata elevata frontem longe impendente, a genis punctatis 

 distinctissima ; margine crasso utrinque bispinoso. Pleurce subplancB, 

 sulcatcE, rectce, apicibus abrupte recurvis. Cauda lata brevis, axi appen- 

 diculato, pleuris primariis latissimis spatulatis, margine postico truncato. 



[Synonym. Acidaspis unica, Wyv. Thomson, Quart. Geol. Journ., 

 vol. xiii. pi. 6, fig. 13.] 



In the absence of a figure sufficiently complete (for the one 

 quoted above is very defective), it is necessary to give a rather full 

 diagnosis of this remarkable form, which tends to show the passage 

 of the Cheirurid into the Aeidaspid family. Indeed, if Prof. 

 Thomson be correct in figuring 12 segments to the body, the species 

 is abnormal for either Acidaspis or Stauroce2ohalus. The shape of 

 the head clearly enough shows that it is to Staurocephalus, or else 

 to one of the sections of Gheiriirus, that this strange fossil must be 

 referred. Cheirurus often has 12 segments, Acidaspis 9 or 10, 

 BtauTOcephalus only 10. The grooved pleurse are unlike Stauro- 

 cephalus, but like the section Eccoptochile among the genus Ghei- 

 ruTUS. But no Cheirurus has so clavate a glabella, though a 

 tendency towards it is exhibited in some species, and Sphcerocoryphe 

 of Angelin is very near to ours. 



There is an evident analogy too in this form with Lichas, both 

 in the shape of the tail and the character of the pleurae. But the 

 external position of the facial suture far up the cheek easily dis- 

 tinguishes it from that genus. I do not further describe it, as it 

 will appear in a very early plate in the volumes of the Palseonto- 

 graphical Society. 



Localities. — Caradoc Schists, at the base of the " Orthoceratite 

 and Graptolite flags," Penwhapple Glen, Ayrshire (W. Thomson). 



Explanation of Plate V. 

 Fig. 1. Staurocephalus Murchisoni, Barrande, coiled specimen, natural size. Wool- 

 hope Shale, Presteign. (Mus, Pract. Geology.) 

 Fig. 2. Head of ditto, same locality and cabinet. 



Fig. 3. Magnified figures from the above specimens, completed from Dudley specimens. 

 Fig. 4 a, b. Magnified specimen from Dudley. Mr. E. Hollier's cabinet. 

 Fig. 5. Lower Silurian specimen. Rhiwlas, Bala (Mus. Pract. Geol.), nat. size and 

 magnified. 



Fig. 6. Staurocephalus globiceps, Portlock (his original specimen, magnified three 

 diameters). 



November 1864. J. W. Salter. 



