PHACOPS (PTERYGOMETOPUS) RETARDATUS. 49 



description of this species previously given by me renders superfluous any remarks 

 on the general characters of the trilobite. 



In proportion to the head-shield the great size of the hypostome is remarkable, 

 the latter measuring 14 mm. and the former only 15'5 mm. in length. The 

 characters of the head-shield in this specimen agree more closely with those of the 

 smaller specimen which was figured [op. cit., fig. 3) than with Mr. Bisset's type; 

 but, as then stated, the small differences scarcely seem to indicate more than 

 'immaturity. The " second" and "third " lateral lobes between the great " cat's- 

 ear " lobes of the glabella are particularly distinctly developed as tubercles, and 

 there is a trace of an intermediate pair of tubercles as in the above-mentioned 

 specimen. 



The hypostome has the typical shape of the genus and is pyriform, the anterior 

 end being wider than the posterior ; the front margin is strongly arched ; the 

 anterior ears are situated at about one-third the length of the hypostome and 

 bend up sharply at the lateral angles of the frontal lobe of the glabella ; the body 

 is gently convex with its sides straight and parallel for its anterior half, but then 

 they sharply converge to the bluntly-pointed tip at an angle of about 45° ; 

 the lateral furrows are weak, and curve gently backwards from the angulations 

 on the sides of the body so as to unite in a continuous curve behind separating 

 off a feebly defined posterior band about one-sixth the total length of the body. 

 The border of the hypostome is narrow at the sides, but widens behind at the tip 

 to about one-fifth the whole length of the hypostome. The marginal furrow is well 

 marked. The whole surface of the hypostome is granulated. In general char- 

 acters the hypostome closely resembles that of Pli. macronr<( as figured by Salter.^ 

 Bimeiisions. — 



Length of hypostome . . . 14*00 mm. 



„ of body of hypostome . . . 11*00 ,, 



Width of hypostome across anterior ears . 11*75 ,, 



,, „ at level of lateral angulations 8*75 ,, 



Length of head-shield . . . 15*50 ,, 



,, of thorax . . . . 21'00 ,, 



„ pygidium .... 14*00 ,, 



Phacops (Pterygometopus) retardatus, sp. nov. Plate VIII, figs. 5 — 7. 



Specific Characters. — Body elongate-oval. 



Head-shield semicircular, twice as wide as long, about one-third the length of 

 the whole trilobite ; median portion somewhat flattened, cheeks steeply bent down 

 and genal angles produced into stout spines. 



Glabella very weakly convex, twice as long as wide at base, subclavate in shape, 

 expanding anteriorly to about twice basal width ; base about one-fourth width of 



' Salter, 'Mon. Brit. Trilob.,' pl.iv, fig. 21. 



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