Vol. 50.] OLENELLTJS-ZONE OF THE N0KTH-WEST HIGHLANDS. 



673 



The absence of faceted pleura shows that these Olenellids had 

 not acquired the habit of rolling up, so that dorsal spines such as 

 those found on Mesonacis and Holmia were probably protective. 

 If so, one asks, what was the nature of the enemies from which 

 these creatures had to defend themselves? We may infer that 



Fig. 2. 



A = Olenellus reticulatus, last two body-segments and telson. 



B = Mesonacis Mickwitzice, eighth body-segment. (After Schmidt.) 



C = Mesonacis asapkoides, thirteenth body-segment. (After Walcott.) 



they were large, for Holmia Broggeri and Mesonacis asajihoid.es are 

 of considerable size. That small enemies preyed upon them, living 

 or dead, is made certain by the occurrence in the collection under 

 review of OZmeZfots-spines which have been bored by some annelid 

 or other animal before fossilization, PI. XXXII. figs. 13-15. The 

 strong spiniform telson may have been used by Olenellus for purposes 

 similar to that fulfilled by the telson in the recent Limuhis. 



The study of these Olenellids plainly shows them to have been 

 very primitive trilobites. They have all their body-segments free. 

 Their glabella is long and cylindrical, and divided into lobes by 

 well-marked furrows. Their eye-lobes are outgrowths from the 

 glabella, and have not wandered far from the primitive axis of the 

 body. There is good evidence for the belief that the occipital or 

 nuchal ring has been once a free segment, and the last to be added 

 to the head-shield, and that the genal spines have travelled back 

 from a more anterior position. 



Of all the Olenellids yet described Holmia (Olenellus) Kjerulfi is 

 the most generalized. Xone of its body-segments are so specialized 

 as to make it conspicuous among its fellows, nor are any of its 

 spines, except the genal ones, more elongated than its neighbours. 

 Its nuchal or occipital segment is much like one of its free segments, 

 but it is fused to the preceding segment instead of being articulated 

 with it. Its genal spines are intermediate in position between 

 those of Olenelloicles and Olenellus, in which respect it is not quite 

 so primitive as the former. From it all the other forms as yet 



