AGNOSTUS RUDIS. 2] 



confused with it so far as tlio head is concerned ; but in this tin; basal lobes arc 



small and tho margin is narrow. 



Of this form very fow specimens seem to have been found in (ireat Britain, and 

 these are mostly heads. In the absence of tails it is difficult to identify the species 

 with certainty; for other forms, such as J. kjeridfi, are barely distinguishable from 

 A.fallax except by the characters of their tail. In Brogger's figure of A. Jejerulji, 

 however, the marginal fold of the head is narrower than in .1. fallax, and in this 

 respect the British specimens which I have seen agree more nearly with the latter. 



Horizon and, Locality. — Menevian : Porth-y-rhaw, St. David's. 



18. Agnostus rudis, Salter. Plate II, ligs. 1:3 — lb. 



18G4 Ayuodus princeps, var. rudis (pans), Salter, Brit. Org. Kern., dw. xi, pt. i, ]>. 4, pi. i, fig. '■> (only). 



Head moderately convex, rounded, with narrow margin which is produced into 

 a short spine at the genal angles. Glabella bilobed ; anterior lobe rather small, 

 rounded in front ; posterior lobe nearly parallel-sided ; basal lobes small. Cheeks 

 smooth, nearly equal in width throughout, confluent with one another in front of 

 the glabella. 



Tail rounded in its general outline, with rather wide margin, which is produced 

 posteriorly into two very small spines. Axis long and rather wide, parallel-sided, 

 rounded posteriorly; anterior segment represented by two small lobes cut off at 

 each anterior corner ; the second segment extends forwards between these two 

 Lobes and posteriorly bears a prominent tubercle; the posterior segment forms 

 nearly two thirds of the axis, is smooth and rounded posteriorly. Lateral lobes 

 very narrow behind axis, where they are confluent with one another. 



Head- and tail-shields, 2 — o"5 mm. long and broad. 



This form is nearly allied to A. dux and A. sidenbladhi. From the former 

 it is at once distinguished by the much greater length of the axis of the tail. 

 A. sidenbladld also has a shorter pygidial axis, and the anterior lobe of the glabella 

 is larger, but less distinctly defined. 



In some respects A. rudis resembles A. pisiformis, and may indeed be looked 

 upon as intermediate between that species and A. dux. But in A. pisiformis the 

 cheeks are separated by a groove in front of the glabella, while the front lobe of 

 the tail-axis is complete across. In A. pisiformis, var. obesus, the first segment of 

 the tail-axis is represented by two small lobes at the anterior corners as in A. rudis, 

 but the axis is much more swollen and the glabella is much longer. 



It may be remarked that in front of the glabella, even in the actual specimens 

 on which Salter's figure is founded, there is usually no trace of the groove which he 

 shows. One specimen, indeed, shows such a groove, but it may possibly be a 

 young .1. trisectus, as it differs also in other characters. 



