BRITISH FOSSILS. 



D 



obscured. But the I. Iceviceps of this author, though not the same 

 as the A s. Iceviceps of Dahnan, is probably a member of the section 

 Gryptonymus, a group which has the axis and glabella lobes well 

 marked out, and often has very prominent eyes. It is very rare in 

 Eno'land and absent in America, but is the common form of the 

 genus in the Swedish area. 



Asaphus Icevigatus, Angelin., Pal. Succ, pi. 29, fig. 1, in many 

 respects resembles A. rectifrons. 



Locality. — Caradoc of Desertcreat, Tyrone. 



o. /. sp. Salter, in Quart. Geol. Journ., vol. vii. pi. 8, fig. 2. A 

 caudal portion of an ud determined species has been figured 

 by Sir E. I. Murchison in his paper on the Silurian Rocks of 

 the S. of Scotland. It is probably distinct from /. gigas. 



4. ? Another is quoted in Prof Nicoi's paper on the Peeblesshire 

 Silurians, Quart. Journ., vol. iv. p. 205, which is lost now, 

 but was stated by myself to be allied to Asaphus {I sot 

 megistos) gigas. Both these species require further illus- 

 tration ; but they seem to show the gradual dying out of 

 the American type Isotelus in its range eastward, as above 

 noticed. Possibly both are referable to Megalaspis. 



The distribution of subgenera over the northern zone is as follows: — 



iV. American Types. British Types. Scandinavian Types. 



Isotelus, common. 



Ptychopyi/e, rare. 



Isotelus, rare, N. and N. 



West only. 

 Basilicus, common, also in 



Mid-Europe. 

 Cryptonymus, very rare, in 



N.W. area. 



[Isotelus, absent.] 

 Basilicus, rare. 

 Cryptonymus, common. 

 Nileus, common. 

 Megalaspis, do. 

 Ptychopyge, do. 



Explanation of Plate III. 



Fig. 1. Asaphus gigas, De Kay, the original specimen figured by Portlock as Isotelus 

 planus, De Kay. It shows the vertical suture to the hypostome and the 

 labrum in place. 



Fig. 2. Do. (Portlock's original of /. ovatus.) 



Fig. 3. Do. (Portlock's figured specimen of /. sclerops.) 



Fig. 4, Tail of do. 



Fig. 5. Cast of under surface of side of head, showing the eye, and the pit for the 



reception of the ends of the pleuras. 

 Fig. 6. Gutta percha cast of the same specimen, showing the real under surface. 

 All the above are from the Caradoc of Desertcreat, Tyrone, and are in the Mus. Pract. 

 Geology. 



J. W. Salter. 



November 1864. 



