240 MESSES. PEACH AND HOENE OX THE 



pressed against the carapace, and shows that it is wider than the 

 dorsal marginal band. No trace of an ' interocular ' spine of Foord 

 is seen on the posterior margin. The chief value of this fragment 

 is to show that the characteristic ornamentation is on so large a 

 scale as to be apparent to the naked eye. The elongation of the 

 pattern on the marginal rim is also very distinctly shown (PL Y. 

 fig. 12 6), and reminds one very much of the characteristic markings 

 of the Asaphidce. 



Portions of test with this characteristic ornamentation, which 

 must have belonged to even larger individuals, occur in the col- 

 lection ; but they are too fragmentary to allow of their being 

 located with accuracy. 



The fossils, other than trilobites, that have as yet been obtained 

 from these dark shales are chiefly remains of pteropods, among 

 which a Salterella like S. j^ulchella occurs. Several species of Hyo- 

 Utiles also occur, together with a flattened curved tube resembling the 

 Helenia hella described by Walcott. One specimen of a large Ento- 

 mostracan, near to Arisfozoe rotundata, is also found in the collection. 



We may be allowed to suggest that the elongated pleural spines, 

 together with the enlarged third segment so pronounced in some 

 species of Olenellus and Paradoxides, are in some way connected with 

 the genital apparatus, and they may be sexual. This may account 

 for their being sometimes present and sometimes absent in the same 

 species. 



One word of speculation as to the systematic position of the 

 Olenellids may be admissible here. The absence of facial sutures 

 suggests that it is from this group that the Limuloids branch off. 

 The ornamentation, the styliform telson, and the small number of 

 body-segments in Olenellus proper appear to indicate a close 

 relationship to the Merostomata through such forms as Stylonurus 

 and Euryptei^us. Olenellus therefore seems to form a central point 

 upon which the more modern trilobites, the Limuloids, and the 

 Merostomata converge. 



EXPLANATION OF PLATE Y. 



Pig. 1. Olenellus Lapworthi. — Carapace, nat. size. From ' Fucoid Beds/ Allt 



Eigh Ian, Dundonnell, Eoss-shire. M 2473''. 

 Fig. 2. 0. Lapworthi. — Carapace, nat. size. M 2468^^. 

 Fig. 2 a. 0. Lapworthi. — Counterpart, nat. size, of fig. 2. Allt Eigh Ian. 

 Fig. 2 b. 0. Lapworthi. — Ornamentation of surface of carapace, enlarged. 

 Fig. 3. 0. Lapworthi, nat. size, from ' Serpulite Grit' near Loch an Nid, Eoss- 



shire. 

 Fig. 4. 0. Lapworthi. — Carapace showing underside, nat. size. ' Fucoid Beds,' 



AUt Eigh Ian. M 24721. 

 Fig. 5. 0. Lapworthi. — Carapace, nat, size. M 2482'^. 

 Fig. 5 a. 0. Lapworthi. — Carapace enlarged, shows glabellar furrows and 



surface ornamentation. ' Fucoid Beds,' Allt Eigh Ian. 

 Fig. 6. 0. Lapworthi. — Portion of carapace, nat. size. ' Fucoid Beds,' Allt EigL 



Ian. M2479'i. 

 Fig. 7. 0. Lapworthi. — Portion of carapace, nat. size. * Fucoid Beds,' Allt Eigh 



Ian. M 2476"^. 

 Fig. 8. 0. Lapworthi. — Portion of carapace, nat. size. ' Fucoid Beds,' near Loch 



an Md. M 2517'^. 



