BY R. ETHERIDGE, JUNR., AND JOHN MITCHELL. 507 



lenses in a vertical row appears to be normal for the examples 

 from the Middle Trilobite Bed; but a specimen from the Upper 

 Trilobite Bed has thirteen to fourteen lenses in the central rows. 

 The tail spine is anchylosed to the border, and when the 

 border and spine are removed a short dagger-like extension of 

 the axis is exposed, such as is shown in most of the figures of the 

 European H. caudatus. It is in this condition that the tail of 

 our species bears a strong resemblance to H. caudatus; but what- 

 ever may be the case in the latter, it is, judging from the evidence 

 furnished by a large number of specimens, almost certain that, in 

 every instance where the tail of our species exhibits the short 

 deltoid form of spine, the true spine has suffered removal. 



We believe the forms figured by McCoy from the Victorian 

 Upper Silurian as Phacops (Odontochile) caudatus to be the same 

 as our H. meridicmus. His figures show the much longer eye 

 and multisegmented pygidial axis: but McCoy's glabell?e are 

 granulate. H. meridianus, both as regards the N.S. Wales and 

 Victorian specimens is so finely granulate throughout as to be 

 practically smooth without a lens. 



Touching the relation of our species to the typical European 

 //. caudatus, Briin., the eyes are proportionatel}^' further forward 

 in H. 7neridianus, the palpebral lobes and genal lobes wider trans- 

 versely, and there is no neck tubercle. The eyes are less lunate, 

 or arched in contour, and consequently longer fore and aft, and 

 the surface of the glabella non-tuberculate. The pygidia differ in 

 the excess of segments over those of ff. caudatus, possessing 

 seldom less than sixteen in the axis of the smaller pygidia, and 

 usually eighteen or nineteen exclusive of the terminal appendage. 

 Victorian and N.S. Wales specimens ^gree in this. Our form is 

 also long tail-spined when perfect, thus resembling /Y. longicau- 

 datus, but unlike the latter we have never seen an individual 

 bearing a frontal spine. As regards the form of the glabella, H. 

 meridianus seems to come nearer to H. longicaudatus. The genal 

 spines are the same length in both the European and Australian 

 forms. //. caudatus occasionally has a granulated pygidium axis, 



