196 Dr. Ivor Thotnas — Trilohite Fauna of Devon and Cornivall. 



agreement with this opinion forms such as Ph. {Trim.) aff. cryptoph- 

 thalmm and Ph. {Trim.) pentops may represent senile stages in the 

 phylogeny of the genus Phacops. On the other hand, the small 

 number of eye facets may be the result of specialization and not of 

 degeneration. 



Phacops (Teimkkocephaltjs) tripaetitcts, sp. nov. PI. YII, Pigs. 4, 5. 



Description. — The smooth cephalon shows a general parabolic outline, 

 and in the case of one specimen has a length of circa 3'3 mm. and 

 breadth of 4 mm. approximately. 



The tumid glabella is regularly curved anteriorly, but somewhat 

 constricted towards the low narrow basal ring, which bears terminal 

 tubercles, one at each extremity. Lateral furrows are not traceable 

 on the frontal lobe. The bounding dorsal furrows are deep, and 

 enclose an angle of about 73°. The ratio of the length to the greatest 

 breadth =9:8. The occipital furrow is fairly broad and almost equal 

 in width to the well-marked and elevated occipital ring. 



The tumid cheelc-areas are triangular, and fall steeply towards the 

 glabella, but more gradually oiitwards towards the marginal border. 

 The latter is a fairly broad and flattened area continued from the 

 rounded genal extremities to the antero-lateral border of the glabella. 

 It is impossible to trace it still further anteriorly in the specimens at 

 present at my disposal. 



JEyes are preserved in one of the specimens, and are each situated 

 upon the extreme anterior corner of the triangular cheek-areas. The 

 facets are circular and few in number. 



Locality and Horizon. — Pour fairly complete heads, together with 

 many fragments, were collected from the same locality and horizon as 

 Phacops {Trim.) aff. cryptophthalmus, etc. 



Pemarks. — This form is closely related to Phacops {Trim.) acuticeps, 

 Kayser, from the Upper Devonian limestones of Martenberge, near 

 Adorf (10, p. 288, pi. xiii, fig. 6). It differs from that species, 

 among other details, in the possession of a straighter posterior margin 

 and less angular genal extremities of the head-shield, in the absence 

 of well-marked lateral furrows on the frontal lobe of the glabella, in 

 the less angular frontal margin, and less continuity in the curvature of 

 the outer border of the marginal area with the frontal extremity. 



A similarity also exists with Phacops{Trim.)micromma, F. A.Eoemer, 

 as interpreted by Dr. Karl Walther (21, p. 317, pi. xv, fig. 6). The 

 German form, however, has well-marked lateral furrows on the frontal 

 lobe of the glabella, and the base of the latter portion is less 

 constricted than in our species. Roemer's figure (15, p. 81, pi. xii, 

 fig. 25) seems very much reconstructed, and is hardly comparable with 

 the British form. 



PEOETID.^. 



Zittel (25, pp. 476, 477) divides this family into the genera Proetus 

 (Silurian, Devonian, and Carboniferous), Arethusina (Silurian and 

 Devonian), Cyphaspis (Silurian and Devonian), Harpides, Cannon, 

 Cyphonisciis (Ordovician), and Phillipsia (Devonian, Carboniferous, and 



