BY 11. ETHERIDGE^ J UNR.^ AXD JOHN MITCHELL. 655 



prominent and important feature. In Encrinurus panctafus, 

 Fletcher described two or three large tubercles, "arranged on 

 each side of the lower half of the glabella.""^ Hawle and Corda 

 figure four pairs in this species, t Angelin four pairs,^ and 

 Schmidt figiu-es five pairs. 1 Under these circumstances, in view 

 of these discrepancies, if the separate descriptions are correct, 

 the number of tubercles in the positions referred to can only be 

 regarded as of specific value. But from the constancy with 

 which the five pairs of axial glabella tubercles occur on all the 

 Australian Eitcriuui-l [except the singular E. duntroonensis^ on 

 which only four are visible, but in this species the fifth pair may 

 be present and hidden by the clasping frontal portion of the free 

 cheek] which have come under our notice, we are inclined to 

 accept Schmidt's declaration of the presence of five pairs in E. 

 ]nmctatiis, as correct. In Australian species, when the free 

 cheeks are in place, it is often difficult to see the anterior pair, 

 and they are still more obscure when the test is also present. 

 Reviewing all the evidence available to us, we must confess to a 

 strong inclination to look upon the presence of five pairs of axial 

 glabella tubercles as probably of generic significance. 



It would appear also to us that these tubercles may mark 

 internal indentations of the cephalic shield made by muscles 

 which operated mandibulary or other appendages of the cephalon, 

 just as in many Pelecypoda the adductor muscles indent the 

 interior of their shells; and when such shells become fossilised so 

 that only internal casts remain, the muscular indents are repre- 

 sented by more or less tubercular-like prominences. 



Along the axial furrows of the thorax, these tubei'cles ai'e even 

 more noticeable features in non-testiferous Austi'alian thoraces, 

 showing up as large tubercular bodies at the outer ends of the 

 axial annulations, but, when the test is preserved, dwindle to 



* Fletcher, Quart. Journ. Geol. Soc, 1850, p.403. 

 t Hawle and Corda, Prod. Mon. l)ohm. Trilobiten, 1847, I'l. v., fig. 55. 



JAngelin, Pal. Scandinavica, 1878, Pt.l, Pl.iv., fig. 4. 

 § Schmidt, Revision Ostbaltischen Sil. Trilobiten, Obth. i., 1881, Pl.xiv., 

 fig.lla. 



