BY JOHN MITCHELL. 1199 



The later portion exhibits few fossils except bryozoan-like 

 markings. Numerous cavities occur, and these were perhaps at 

 one time occupied by corals. The composition is shaly, and its 

 thickness 20 to 30 feet. 



Next in order comes a bed of slightly hardened greyish shale, 

 40 to 50 feet thick at the point where J have made the section. 

 In other places it reaches double of that thickness. This may be 

 designated the ' lower trilobite zone,' for it is here that this order 

 first appears as a distinguishing feature. In its eastern out-crop 

 representatives of the following genera occur : — Acidaspis, Cromus 

 ( Beaumonti), Eiicvinurtis (punctatus), Ccdymene, and Sphcerex- 

 ochus. From the western outcrop, in addition to the above 

 genera, it has yielded Harpes, Bronteus, Cheirurus, Stauro- 

 cephalus, Proetits, and others. The mollusca are represented, with 

 few exceptions, by brachiopods of small size. Prints of hydrozoa 

 are plentiful ; and occasionally the remains of a seaweed may be 

 found. One of the most noteworthy things in connection with this 

 bed is the sudden appearance of so many trilobites, and particularly 

 those belonging to the genus Acidasp)is, which in this case follows, 

 instead of precedes, such genera as Encrinurus, Cheirurus, and 

 Cromus. This bed came to an end by the cessation of sedimentary 

 deposits, and a comparatively clear sea supervened for a short period, 

 and admitted of the formation of a thin bed of impure limestone 

 apparently not more than 10 feet thick. This is the ' Phacops 

 Bed' of Mr. Charles Jenkins, (vide his paper on the Geology 

 of Yass Plains). It is worthy of note that below these Calceola 

 sandalina occurs ; also a small coral resembling Petraia bina. The 

 whole bed largely consists of trilobite remains arranged in layers. 

 The sea must have literally swarmed with them. But notwith- 

 standing their number, to obtain a perfect specimen of any species 

 is a rarity. Among the genera that can be distinguished are 

 Phacop>s, Acidaspis, and one resembling Angelina. The largest 

 species has, from the inspection of a fragment only, been pronounced 

 to be Phacops longicaudatus. With Mr. Jenkins I consider that 

 it bears a strong likeness to some forms of Dalmania. 

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