BIJITISH FOSSILS, 



EXPLANATION OF PLATE X. 



Fig. 1. Figure of the entire trilobite, slightly restored, from the Builth specimen. 

 All the parts are shown in specimens in the collection of the Geological Survey. 



Fig. 1*. Dissection of the species. Lines as if of a facial suture are seen ou the glabella, 

 but they do not seem to indicate any true separation of parts. 



Fig. 2. An individual still younger. 



Fig. 3. A small specimen, showing the constancy of its characters at an early stage. 



Fig. 4. Granulation of the head. 



Fig. 5. Granulation as seen towards the extremity of one of the pleurae. 



Fig. 6. Lineation of deflected surface of tail margin. 



Note on the British Species of Ampyx. 



The genus Ampyx was first recognized as British by Lieut.- Col on el Portlock, R.E., who 

 described and figured two species, Ampyx Austinii, identical with A. mammillatus (Sars), 

 and Ampyx Sarsii, identical with A. roslratus (Sars), and probably with A. nasuttts (Dai- 

 man). Both these forms were discovered during the researches of the Ordnance Geolo- 

 gical Survey in Ireland, the former in the Silurian slates of Tyrone, and the latter in 

 those of Newtown, near Waterford. Recent researches have led to the conclusion that 

 the strata in which these fossils were found are members of the lower (Llandeilo or 

 Bala) division of the Silurian rocks. 



In Great Britain neither of these species of Ampyx have, as yet, occurred. The only 

 species described as British, besides that now figured, is my Ampijx parvulus, from the 

 Lower Ludlow of Vennal Hill (Mem. Geol. Surv., vol. ii. pt. 2, p. 350, pi. x.) There 

 appears, however, to exist a third species in the Rhiwlas limestone, near Bala ; one dis- 

 tinct from any as yet described, but of which only heads and tails have been found. The 

 largest head is half an inch in length, exclusive of the spine. It is nearly allied to A. ros- 

 tratus, but differs in having much smaller cheeks in proportion to the glabella, whioh is 

 even more produced than in the rostratus, and exceedingly tumid. The cheeks in the 

 species to which I have compared it are beyond the middle of the glabella ; in the Rhiw- 

 las form never so far as that part. The neck lobe of the latter is narrow, and the tail, 

 though similar in sculpture, much shorter and wider. The proportions of the cheeks and 

 glabella remind us of A. Bruckneri (Boll in Dunker and Von Meyer's Palseontogra- 

 phica, 1st Band, 11 lief., t. xvii., f. 8), but that species has a carinated glabella, and very 

 broad neck lobe. Ampyx Poitlockii (Barrande), is said to have the glabella much less 

 elongated than A. nasntus, and therefore cannot be the Rhiwlas form. A. bohemiciis 

 (Hawle and Corda, t. 3, f. 19), comes nearer, but is represented as having a much wider 

 glabella and a proportionately much smaller tail, with no traces of rings on its axis. I 

 propose to name the Rhiwlas species, Ampyx tunridus. 



Ampyx baccatus, of Portlock, does not belong to this genus. 



E. FORBES. 

 July, 1849. 





