108 SYSTEMATIC ARRANGEMENT 



suddenly deflexed, not produced. The glabella distinctly defined, broadest at the anterior, 

 narrowing towards the posterior part, contracted in the shape of a peduncle before 

 the margin of articulation, and there elevated in the centre into a protuberance ; beside 

 it, at each side, another more level protuberance, which extends to the eye. A deep fur- 

 row separates the margin of articulation from the cephalic axis, and the posterior half of 

 the lateral surfaces from the other surface ; it disappears, however, towards the obtuse, 

 rounded, posterior angle. Eyes short, but prominent. 



Axis of the body moderately arched, broader towards the centre than at either end, the 

 separate rings strongly arched. 



Caudal shield at the base broader than long, rather obtuse at the end, moderately con- 

 vex, the axis even at the commencement rather narrower than the last ring of the body^ 

 obtuse at the posterior part, articulated anteriorly, but more or less distinctly so, (which 

 depends on the size of the individual,) eight distinct rings in all, rarely more; the sides with- 

 out ribs. The whole upper surface of the shell is not smooth, but covered with elevated 

 fine ridges, which run obliquely towards the external and posterior part ; between them 

 are impressed points or dots, which are occasionally united into spiral lines; there are 

 usually from seven to eight larger striae on the sides of the caudal shield, which correspond 

 to the joints of the axis. Badly-preserved specimens are so worn that they appear to be 

 smooth. 



Remark. — Perfect specimens are seldom more than three inches long, but caudal shields of much 

 larger individuals have lieen fouud, especially in boulders (as in the collection at Halle). These {Tril. 

 Schrosteri seems to be the same species) must have attained the length of six inches.* 



5. Asaphus tyr annus : Protuberantia capitis ovata ; angulis scuti cephalici posticis caudEeque acutis 

 vel acuminatis. Long. 6-10". Tab. V, Fig. 4. 



i?(/.— MuRCHis. Sil. Syst. ii, 662, PI. 24. Emmr. Dissert. 29. 6. Milne Edw. Cr. 

 iii, 310. 7. 



Locality. — The Llandeilo flags of England, in Caermarthenshire, Pembrokeshire, and in 

 boulders in a red limestone in the collection at Halle. 



The cephalic shield is not yet sufficiently known ; but Murchison's figure leaves no 

 doubt that the facial suture described a circle at the anterior part, and that the posterior 

 angles were much produced. 



The large caudal shield, which I have represented, belongs undoubtedly to this species, 

 and shows that Murchison's figure of its extremity represents it as rather too pointed. The 

 sculpture of the upper surface, according to Murchison's figure, appears to be exactly the 

 same as in the preceding species, only less delicate ; it is wanting in my specimen ; I have 

 copied Murchison's figure in this respect. 



SuMirisioH B. — The facial suture describes an angle at the anterior part, and is not 

 semicircular. 



a a. — The caudal axis projects, and is distinctly separated by a furrow in the shield. — 

 Asaphus. 



* Tlie large caudal shields, of which I am here speaking, belong to Isoteles Powisii, Portl. {Rep. 

 of Geo/. 297, PI. YI, Fig. 1), and must not therefore be identified with Asaphus ex'jiansus. I now very 

 much doubt whether Tri/. Schrceteri, Schloth. belongs to it. 



