Vol. 50.] OLENELLUS-ZONE OP THE NORTH-WEST HIGHLANDS. 671 



Genus Bathynottts, Hall. 



Bathtnottjs holopygia? Hall. 



Some fragments consisting of a portion of a glabella and fixed 

 cheeks, and several slender spines of trilobites which cannot well 

 belong to any known species of Olenellus, but which fairly answer 

 to the description of parts belonging to Bathynotus holojnjgia, occur 

 in tbe collection under review, and have been provisionally named 

 as above till such time as further evidence regarding their nature 

 is forthcoming. 



III. Theoretical Considerations based upon the Study of 

 the Remains described. 



Having described these trilobites, I may now proceed to compare 

 them among themselves and with other known Olenellids, and 

 endeavour to correlate their homologous parts. In all the Olenellids 

 the glabella and eye-lobes are so similar, that no difficult} 7 arises in 

 correlating part with part. It is not so with the marginal spines 

 of the head-shield. The spines upon the pleural angles of the 

 posterior margin in Olenellus intermedins, PI. XXXII. fig. 7, and 

 so strongly pronounced in Olenelloides, PL XXXII. figs. 1-6, are 

 represented in the other forms in the collection by the rounded-off 

 pleural angles. They have been shown by Pord and Walcott to be 

 present in young stages of the American Mesonacis {Olenellus) asa- 

 phoides, PL XXXII. fig. 11, and to disappear into the rounded 

 pleural angles in the adult. They are present as spines in Holmia 

 (Olenellus) Kjerulfi, PL XXXII. fig. 12, and in the other described 

 species of this genus. A study of fig. 12 (after Holm) reveals that 

 they are, in all probability, the pleural spines of a segment com- 

 parable with one of the free body-segments, the traces of the axis 

 and pleura of which have not been obliterated by its fusion with 

 the other segments of the head-shield. 



The genal spines of most Olenellids occupy the postero-lateral 

 angles of the head-shield. That this was not their original position 

 is made almost certain by the researches of S. W. Ford and Walcott, 

 who have shown that in some of the young stages of Olenellus 

 Gilberti, PL XXXII. fig. 9, these spines may be produced in a line 

 with the anterior margin, and that they travel round as the animal 

 gets older, PL XXXII. fig. 10. In Olenelloides, PL XXXII. 

 figs. 1-6, we have an adult form which shows these spines placed 

 about halfway between the anterior and posterior margins. That 

 they have travelled back from a more anterior position is rendered 

 probable by the margin behind them being concave. If that be the 

 case, the notch between the genal spines and the pleural angles 

 in Olenellus, Holmia, and Mesonacis represents what has once been 

 a lateral margin. This inference is supported by evidence gained 

 from the study of the ornamentation on Olenellus. In the paper 

 read before this Society in 1892 I pointed out that the polygonal 

 pattern of the test becomes highly elongated on the thickened 



