8 



BRITISH FOSSILS, 



AGHOSTUS TRIKODUS. 



Plate I. Figs. 8—10. 



Diagnosis. A. brevis, semhtncialis. Caput suhorbiculare, glcibeMa 

 convexa, nec ultra f capitis extensa, Integra; limbo convexo. Cauda 

 transversa^ convexa; axi miruto conico, vix climidium caudce ej^ciente ; 

 titrinque biloha, tuberculoque magno. Lirabus posticus convexus, a margine 

 hispinoso et ab axe profunde sejunctus. 



Synonyms. Trinodus agnostiformis, McCoy, Sil. Foss. Irel., pi. 4, 

 fig. 3 ; in PaL Foss. Woodw. Mus., t. 1 E., figs. 12, 13 (not fig. 11) ; T. 

 tardus, ib., fig. 9 [^A. glabratus, Angelin, Pal. Suec, t. 6, fig. 5]. 



The original fignre in the Memoirs of the Survey was not quite 

 satisfactory, for the tail segment (from decomposing limestone near 

 Haverfordwest) had lost its axis furrows and its marginal spines. 

 It is replaced by better specimens in our Museum from Ayrshire. 

 The head has been found more abundantly than the pygidium, and 

 is at once distinguished from all other British published forms 

 (except the kindred species, A. limhatus) by the simplicity of its 

 glabella, in which, and many other points,, the species closely re- 

 sembles A. tardus, Barrande. 



There was a wrong reference in my first description of this 

 species, for I quoted Professor McCoy's synonym for the A. lim- 

 hatus next described, regarding both as varieties of the A. trinodus, 

 and giving the present one the varietal name, /3. convexus. McCoy 

 corrected this error in the Cambridge work^ where he again 

 figured two imperfect heads of the species. I hope now to have 

 remedied all our deficiencies by these excellent specimens, chiefly 

 obtained by Professor Wyville Thomson from Ayrshire. 



Description. — A small species, not above five lines long and 

 three lines broad, very convex for the genus. The head and tail 

 rounded, with a narrow equal border all round, very distinct and 

 separated by a sharp sulcus. In the head the limb is equal in 

 breadth in front and on the sides, separated by a sharp line from 

 the short parabolic glabella, which has no lobes or furrows of any 

 kind. It is a little constricted about the middle, and is rounded at 

 its base, just above a pair of small transverse basal lobes. The 

 posterior angles are contracted and minutely mucronate (fig. 8 a). 



The body rings are not known. 



The tail (fig. 10, 10 a) is semioval, the upper angles not contracted 

 like the base of the head, but rectangular. The central lobe is 



