BRITISH FOSSILS 



Decade VII. Plate I. 



PHACOPS DOWNINGI^. 



[Genus PHACOPS. Emmrich. (Sub-kingdom Articulata. Class Crustacea. Order 

 Entomostraca. Tribe Trilobitse or Palseadae.) Head strongly trilobed; glabella lobed, 

 and broadest in front ; facial suture ending on the external margin ; eyes largely 

 facetted; hypostome oblong, rounded at the end; thorax of 11 segments, the pleurae 

 grooved and facetted for rolling up ; tail strongly ribbed, of several segments, the margin 

 entire or toothed.] 



[Sub-genus Acaste. Goldfuss. Form convex, and contractile into a ball. Glabella 

 not much inflated, all the lobes distinct ; facial suture within the edge or marginal in 

 front ; head angles rounded or with short spines ; hypostome obtuse, entire ; body 

 segments rounded at the ends; tail of a moderate number of distinct segments (11 or 

 less), its edge without lateral spines,] 



Diagnosis. -P. alutaceus; copite transverso^ margini frontali angulato ; 

 glabella depressa oblonga subparallella, sulcis utrinque tribus distinctis, lobo 

 basali lineari, secundo ovali, superiori transverso — sed margine superiore 

 ascendente sinuato — lobis omnibus planis et fere ad medium glabellcE extensisy 

 spatio angusto interjecto : lobo cervicali elevato ; oculis magnis nec eminenti- 

 bus : Cauda subirigona, axi convexo costis quinque distinctis tribusque obscuris 

 prmdito, — lateribus quinque- costatis, costis duplicatis ; margine distincto, 

 apice angulato. 



Stnonyims. Calymene macrophthalma (Brongn.), Buckland (1836), 

 Bridgw. Treatise, pi. 46. fig. 5 (not 4.) Calym. ? Doivningice, Muechison, 

 Silur. Syst, (1839), pi. 14. fig. 3. Milne Edwards (1840), Crust., 3. 324. 

 Acaste Downingice, Goldfuss, Syst. Uebersicht der Tril., Neues Jahrb. 

 (1843), 563. Phacops macrophthalmus, Burmeist. (1843), Org. der Tril., 

 139, 140, and in ed. 2. (1846), p. 92. Phacops DowningicB, Emmrich, 

 Neues Jalirb. (1845), 40. pi. 1. fig. 2. [icon mala] ; Transl. in Taylor's 

 Scient. Memoirs (1845), vol.iv. pi. 4. tig, 2. Phillips and Salter, Memoirs 

 Geol. Surv. (June 1848), vol.ii. pi. 1. p. 239, 336. pi. 5. fig. 2. 3. 4. M'Coy 

 (1851), Synopsis Pal. Foss. Woodw. Mus. 160. 



Junior. — Asaphus subcaudatus, and A. Cawdori, Muechison, Sil. Syst. 

 pi. 7. fig. 9, 10. Phacops subcaudatus, Salter and Phillips, 1. c. 239. 



One of the most common, and certainly one of the most elegant 

 trilobites in the Silurian System — occurring in abundance wherever 

 Upper Silurian strata are found. It is a very characteristic fossil of 

 [vir. i.] 7 A 



