PHACOPS. 35 



This is not a difficult species to recognize, once its peculiarities are mastered ; but it 

 occurs in a variety of forms, and these are rendered more obscure by the changes the 

 rock has undergone. It is common in the sandy schists of Tyrone, and not unfrequent 

 in North Wales and other places. Colonel Portlock thought it identical with the P. 

 macropJtthahna of Brongniart and the P. Downingice of the Wenlock Rocks; and to 

 avoid the confusion of using these controverted names, proposed to term the combined 

 species P. Brongniarti, after the author of the ' Crustaces Fossiles.' 



But, as I have shown in the Decades of the Survey (/. c), these are really three very 

 distinct species, and we must limit Portlock's name to the species described first by him. 

 His P. Mt/rchisonii is only a synonym. But it is with some little doubt I include the 

 P. Dahnani, a form with rounded front, but otherwise exactly like the species with 

 which it is associated. As I find Welsh and Irish specimens with an intermediate 

 character, I do not think it can be wrong to unite these two, and I take the pointed form 

 to be the <$ , and the more rounded variety (P. Dahnani) to be the ? form. The <$ is 

 the more common of the two. 



Two inches long, elongated, and tolerably convex, the head occupying less than one 

 third the whole length ; subtriangular, and with the front produced and recurved (in the 

 P. Brongniarti $ ; semioval and with a rounded front in the ? form, P. Dalmani ,-) 

 the glabella itself is of the same shape in both varieties, tubercular all over, contracted 

 at the base, expanded in front, but not convex, and with radiating lobes. The forehead- 

 lobe is subrhomboidal and transverse, the upper lateral lobe large, triangular, and over- 

 hanging all the rest, but not so large as to lender them obsolete. The mid-lobe is 

 directed backwards, and is parallel-sided, and about half the length of the front one. 

 The basal lobe is transverse-linear, contracted at the sides, where it forms a tubercle, and 

 the neck-lobe is again broader and thicker and more elevated than the rest. This is less 

 conspicuous in the ? form. The cheeks are triangular, and concave outside of the very 

 large eye, which reaches forward beyond all the lateral lobes, and backward to the neck- 

 furrow. It is considerably arched, rather depressed, and has numerous small lenses 

 (about eight in a vertical row, and in all 170 in each eye). The eye-lobe or -lid is strongly 

 furrowed parallel to the lens-bearing surface. The space between the eye-lobe and the 

 glabella is convex, and of an oblong shape. Angles of the cheeks obtuse, not spinous. 

 Thorax with a convex, narrow axis, and with pleurse which are steeply bent down at 

 about the first third of their length ; the fulcrum placed within the middle point ; the 

 ends of the pleural blunt and arched forward, and their facets large. 



The tail is triangular, pointed in the <$ , obtuse in the ? form, rather convex ; with 

 a long conical axis reaching nearly to the narrow margin, and interlined strongly by 

 intermediate furrows, which project beyond them and quite reach the margin, undulating- 

 it most strongly in the S form. 



I see no reason for separating the two trilobites quoted above as specific forms. All 

 the characters are more strongly pronounced in the form which I suppose to be the S > 



