THENTON LIMESTONE. 247 



304. 1. PHACOPS CALLICEPHALUS. 



Pl. LXV. Figs. 3 a- 1. 



Compare Phacopa Dalmanii, Poktiock, Geol. Rep. Londonderry, pag. 282, pi. 2, fig. 7 a, ft, c ; and pi. 3, fig. 7 d. 

 Also Phacopa Murchisonii, pi. 2, fig. 9 a, ft; and pi. 3, fig. 9 c, d. 



Buckler somewhat semicircular or sublunate, convex, produced at the posterior angles 

 into a broad rounded wing-like expansion ; glabella broad and rounded in front, narrowing 

 posteriorly, and. three-lobed upon each side, the lower lobe short, with a tubercle at the 

 base, or between it and the posterior angle of the eye ;■ anterior angle of the eye reaching 

 to the groove between the frontal and second lobe of the glabella ; eyes lunate, reticulated, 

 elevated to the same line as the most prominent part of the glabella ; thorax with eleven 

 articulations, the lateral ones grooved ; caudal shield rapidiy tapering, with fourteen or 

 fifteen articulations in the axial lobe, and nine in each lateral lobe ; the latter are distinctly 

 grooved, and all terminate in a smooth border. Surface of the buckler pustulose-punctate ; 

 of the articulations, less distinctly marked. 



This is a beautiful little species, bearing a very close resemblance to the Phacops dalmanii 

 cited above ; the only essential difference being in the small tubercles between the last 

 lobe of the glabella and the posterior angle of the eye, which are not mentioned in that 

 one, and the number of articulations in the caudal shield, which are, in Portlock's species, 

 nine in the central, and six in the lateral lobes. 



The surface of the buckler is very beautifully marked with small rounded pustules, the 

 surface of which, and the spaces between, are finely punctulate ; differing in this respect 

 from any other species. The axial lobe of the thorax has the surface marked with elongated 

 or irregular pustules, with fine punctse ; while the lateral lobes are more finely papillose 

 or granulate, and the same characters are more or less distinct in the caudal shield. It is 

 the only species of the genus known to me in our lower strata, and differs from Phacops, as 

 restricted by Emmeich, in having more than eleven articulations in the caudal shield. It 

 must be regarded, however, as one of the earliest representatives of that type, which 

 afterwards became more numerous, and varied in form, in the higher palaeozoic strata. In 

 this species the thorax and caudal shield have the characters of Calymene, and it would be 

 difiicult to determine from these alone the true nature of the fossil. The eyes are as distinctly 

 marked as in any of the subsequent species, and the direction of the facial suture is the same. 



Fig. 3 a. Cephalic shield of this species. The specimen is well preserved, and shows the extension of 



the posterior angles in a very perfect manner. 

 Fig. 3 A. A small entire specimen of the same species. 

 Fig- 3 c. The caudal shield, showing the number of articulations, etc. 

 Fig. 3 i. A magnified portion of the surface of the buckler, showing the pustulose-punctate character 



of the surface. 

 Fig. 3 e. A magnified portion of the articulations of the thorax, showing the irregular pustulose surface. 

 Fig. 3 / Lateral view of the eye of this species. 3 g. Magnified view of the eye. 

 Fig. 3 A. A contracted specimen. The lobes of the glabella are obliterated, and the surface of the eye 



