OLENELLirS ZONE IN THE NORTH-WEST HIGHLANDS. 235 



crowded with SerpuUtes Maccullochii (Salterella) and traversed by 

 vertical worm -burrows, projecting as long pipes, several inches in 

 length, on weathered surfaces. 



The small development of the Durness Limestone represented 

 in the Duudonnell Porest belongs to the base of the Ghrudaidh Group. 

 The bands immediately overlying the ' Serpulite Grit,' are richly 

 charged with serpulites like the corresponding beds in the Eriboll 

 sections, a sub-zone which has been recognized along the whole line 

 from Eriboll to Loch Kishorn. 



The appearance of serpulites (Salterella) on these various horizons 

 ranging from the ' Pipe-E-ock ' to the basal limestone, and their asso- 

 ciation with Olenelhcs in the ' Pucoid Beds ' and ' Serpulite Grit ' lead 

 us to cherish the hope that portions of Olenellus may yet be found in 

 certain shales in the quartzites and priDbably in the lowest group of 

 limestone. 



§ 7. CONCLFSIONS DEAWN FROM THE DlSCOV^ERY OF OlBNELLUS. 



The evidence now adduced proves (1) that the ' Pucoid Beds ' and 

 ' Serpulite Grit ' are of Lower Cambrian age, the quartzites forming 

 the sandy base of the system ; (2) that the Torridon Sandstone, which 

 is everywhere separated from the overlying quartzites by a marked 

 unconformability, is pre-Cambrian. 



§ 8. Description of Possil Bemains. 



Of the organic remains obtained from the dark shale-bands 

 mentioned in the former part of this paper, fragments of trilobites 

 are the most abundant, and the portions most commonly met with 

 are their carapaces.^ 



Prom a collection of over fifty specimens there are only three that 

 show any other recognizable part of the animal. One is a body- 

 segment, either the first or second ; another, from its greatly de- 

 veloped recurved pleural spine, may be referred to the third segment ; 

 while the remaining one appears to be a portion of a styliform 

 telson. 



The form of the carapaces, the arrangement of the glabella, the 

 palpebral lobes and eye-slits, the cheek-spines, and the absence of 

 any true facial suture, indicate that these remains are referable to 

 the genus Olenellus and to that section of it characterized by 0. 

 Thompsoni of Hall. 



With the exception of a few portions of a large species too frag- 

 mentary for description all the remains may be ranked under one 

 species, which however shows a considerable range of individual 

 variation. 



The fragments of body-rings and telson above alluded to appear 

 to belong to different individuals and cannot be referred to any one 



^ [In the present paper I use the term ' carapace ' foi- that portion of the test 

 .of the trilobite which is homologous with the shield bearing the eyes and 

 covering the six pairs of limbs in Limulus, Scorpio, Slimonia, Kurypterus, and 

 Pterygotus.—B. N. P., Feb. 26th, 1892.] 



