BRITISH FOSSILS. 3 



nearly five -twelfths. The breadth of the thorax was one inch. The 

 length of a cheek-spine one inch and five-twelfths. 



Affinities. Its nearest ally is the Ampyx mammillatus of Sars (A, 

 Austinii of Portlock), which it resembles in general outline, in form of 

 head, and features of glabella, and also in the number of thoracic rings. 

 The tail of Ampyx mammillatus is, however, very differently shaped, and 

 has only one lateral furrow on each side, and its axis is, in great part, 

 smooth. There are also very material differences in the structure of 

 the cheeks and pleurae. Ampyx nasutus (Dalman), is described as 

 having six articulations, but has a very prominent glabella protruding 

 much beyond the cheeks, and a tail represented as smooth at the sides. 

 The figure given by Dalman, however, so closely resembles the Ampyx 

 rostratus of Sars (A. Sarsii, Portlock), that I am inclined to think that 

 the fold on each side of the tail above the single lateral furrow was 

 mistaken for a joint of the thorax, for in all the other species of Ampyx 

 there are only five thoracic segments with the peculiar triangular outline 

 of the head, which is so shaped in consequence of the great projection of 

 the front of the glabella beyond the cheeks. These characters we see 

 combined in Ampyx rostratus, Ampyx Portlockii (Barrande), Ampyx 

 bokemicus of Hawle and Corda, and my Ampyx parvulus. The Ampyx 

 Brucltneri of Boll has the same form of head, but the body of that 

 species is unknown. If, in the end, all the five-ringed species should 

 prove to have long heads, and those with six thoracic segments to have 

 short and rounded ones, the latter section may be conveniently distin- 

 guished from the former as a subgenus, under the name of Brachampyx. 



History. This curious trilobite was discovered by Sir Roderick 

 Murchison, and figured by him in the Silurian System as a species of 

 Trinucleus^ of which genus he considered it a member, supposing the 

 border of the head to be lost, and regarded it as allied to his Trinucleus 

 fimlriatus, but as differing in the number of ribs on the tail. In 1845, a 

 number of specimens were found in the original locality at Builth by 

 Sir Henry de la Beche and myself, in more perfect condition than those 

 first figured, and they proved, without question, to belong to the genus 

 Ampyx. On examination of the original specimens figured in the Silu- 

 rian System, I found that they also exhibited traces of the frontal spine. 

 The true generic position of this trilobite was afterwards indicated in a 

 note to the Ray edition of " Burmeister's Essay ;" that author had re- 

 garded it as a mutilated example of Trinucleus fimlriatus. 



British Localities and Geological Ranges. Hitherto this Ampyx has 

 occurred only in the LLANDEILO FLAGS of Carneddau Hill, near Builth, 

 Radnorshire, South Wales. 



