662 



TRILOBITES OF THE LOWER SILURIAN ROCKS. 



tail. Caudal portion broad and very obtuse, lateral lobes convex, ribs 10 ? each having an 

 additional furrow near its posterior edge. (The specific name is taken from the last-men- 

 tioned feature.) 



Loc. Wilmington near Marton, Salop. In sandy, black shale which rises from beneath the 

 Upper Silurian Rocks of the Long Mountain. A rare species found by myself. 



TRILOBITES OF THE LLANDEILO FLAGS. 



PLATES XXIV. AND XXV. 



Asaphus Buchii, PI. 25. f. 2. Brongn., PI. 2. f. 2. a, b. 



i( Corpore ovato, antice obtuso ; pars caudse membranacea ad marginem longitudinaliter 

 striata." — Brongn. 



M. Brongniart further describes this Asaphus, as having an ovate form, the head being at 

 the larger end; the central lobe of the buckler pretty distinct, and terminating anteriorly in 

 a slightly tuber culated point. 



The lateral lobes of the head described by Brongniart as triangular, are more neatly defined 

 in our specimen, and show a curved outline of the posterior margin, the eye being placed near 

 the lobe centre of the f . 2 a and 2 b. In Brongniart's specimens the outer rim of the buckler 

 is indistinctly marked. In our figure it is seen to advance beyond the centre of the body, and 

 to terminate in a fine point. The costal arches near the ends of the lateral lobes of the body 

 are double (see f. 2 a.) The central lobe is nearly pyramidal. The coriaceous membrane 

 which covered the ends of the caudal ribs, is striated parallel to the margin. The transverse 

 studs, marking the prolongation of the caudal ribs beneath the coriaceous membrane, is a 

 character much insisted on by Brongniart, PI. 2. f. Sb l . I may here observe, that after the 

 plates had been engraved, Lord Cole lent me a specimen of Asaphus Buchii (from Builth), 

 rather larger and having a more perfect head and buckler than that which I have figured (from 

 the collection of Mr. C. Stokes) ; this specimen further exhibits the termination of the body 

 ribs in deflected points, and the fine striation of the external coriaceous membrane is beautifully 

 exposed. The specimen figured is from the cabinet of Mr. Stokes. 



Loc. Abundant throughout the Llandeilo formation, portions of it occurring in numberless 



quarries in Caermarthenshire, Pembrokeshire, Radnorshire, Brecknockshire, Shropshire, 



east flank of the Berwyns, fyc. 



It is said to occur in Norway, Russia and America. 

 Asaphus iyrannus, n.s. PI. 25. f. la b., and PI. 24. 



Spec. Char. — Head, posterior margin of buckler extending in a short spike to the third seg- 

 ment of the body. Body, central lobe very broad, and contracting suddenly as it passes into 

 the caudal portion. Ribs 8, subdivided by intermediate plates ivhich terminate obtusely. 

 Caudal portion elongated and pointed ; ribs deflected to the tail at an acute angle. 



This trilobite thus differs essentially from Asaphus Buchii. 



The specific distinctions are all visible in the figures, though the peculiar form of the head 

 and buckler of Asaphus tyrannus is still better seen in an unfigured species recently sent to 

 the British Museum by the Earl of Cawdor. 

 Asaphus tyrannus, varietas ornata, PI. 24. 



I venture to consider the forms in PI. 25. f. 1 ?, and PI. 24., as belonging to the same species, 



