OLENELLUS ZONE IN THE NOKTH-WEST HIGHLANDS. 239 



both margins by a thickening, the anterior one being the stronger ; 

 it ends in a short pleural spine bent backwards at nearly a right 

 angle to the pleuron. A groove passes obliquely from the angle 

 made by the anterior margin with the edge of the middle lobe to 

 the postero-lateral angle. 'No tubercle is seen on the middle lobe. 



A body-segment with central lobe, 7 mm. long by 3 mm. deep, 

 with left pleuron attached 7 mm. long (PI. V. fig. 10), curves off 

 into a greatly elongated pleural spine 15 mm. in length. A similar 

 groove to that already described crosses the pleuron obliquely and 

 is continued into the spine. Prom the great length of the spine 

 it is probable that this is part of the third body-segment. No other 

 segment has been observed. 



Telson. — A portion of a styliform telson 12 mm. long, with an 

 articular surface 5 mm. wide at the anterior end and 4 mm. wide at 

 the broken extremity, is preserved as an isolated fragment on a slab 

 with the remains of this species (PI. Y. figs. 11 & 11 a). 



On every portion of the upper surface -the test is ornamented by 

 a network of raised lines which enclose polygonal spaces (PI. Y. 

 fig. 2 h). The nature of this ornament is the same throughout, but 

 the pattern is varied according to position — the spaces becoming 

 more elongated towards the lateral margins. In the present species 

 it is most pronounced upon the medial lobes of the glabella, where it 

 can be seen by the use of a strong lens. On an average three of 

 these spaces measure about one millimetre.^ 



The above description shows that the present species is very near 

 to Olenellus TJiompsoni of Hall. It differs from that species in 

 being smaller, in the glabellar furrows crossing the glabella, in their 

 general arrangement, and in the presence of a rudimentary mesial 

 spine on the occipital ring. 



Erom 0. Gilherti it differs in the posterior angle of the palpebral 

 lobes being removed from the edge of the glabella, and from 

 0. Iddingsi and 0. Walcotti in the form of the carapace. 



It is named after Prof. Chas. Lapworth, P.R.S., who has done so 

 much towards the elucidation of our older Palaeozoic rocks and who 

 was the first to prove the existence of Olenellus in our own country. 



Among the disjecta membra of Olenellid trilobites from the dark 

 shales above described are certain portions of the carapaces of a much 

 larger species than that described. Two of these fragments allow of 

 the part of the animal to which they belong being made out. The 

 first, represented in PL Y. fig. 12, natural size, is part of the left 

 angle of a carapace with the genal spine. It bears a broad marginal 

 band, and the genal spine is short and broad. The width of doublure 

 is also indicated by the bulge its inner margin causes by being com- 



^ An ornamentation similar to this has been described by Walcott as occurring 

 in 0. (Mesonacis) asaphoides, Emmons, and also in 0. {Holmia) Broggeri, and 

 by Schmidt in 0. {Mesonacis) MicJcwitzi. — Dr. Hicks, in his article on ' The 

 Fauna of the Olenellus Zone in Wales' in Greol. Mag. for 1892, p. 21, 

 mentions the occurrence in the St. David's rocks of fragments of a trilobite 

 which 'show a reticulated ornamentation.' These fragments he ascribes to 

 Olenellus. 



