114 HORN EXPEDITION — PALAEONTOLOGY. 



There are recognised two groups of rocks,* (1) The "Auriferous Slate Group" 

 in the north-east, and (2) "The Gordon River Group." Tliese two groups do not 

 come in contact, but from fossil evidence the Gordon Group is considered to 

 succeed the former. The Auriferous Slate Group contains the remains of 

 graptolites and a small species of Orthis, and it may tentatively be regarded as the 

 equivalent of the Graptolite Slates of Victoria. 



The facies of the MoUuscan fauna and the lithological characteristics of the 

 Gordon River Group ai'e in accord with the Larapintine Ordovician, and in marked 

 contrast with the Victorian type. The absence of Trilobites?, which serve to fix the 

 age of the Larapintine beds, and other salient forms in the Gordon River Group, 

 may raise a doubt as to their synchronism. As to whether the Gordon River 

 Group and the Larapintine Series exhibit any closer affinity in their faunas than is 

 indicated by the generic grouping of their mollusca cannot be ascertained except 

 by comparison of actual specimens. With few exceptions the fauna of the Gordon 

 River Group has not been diagnostically made known. Mr. Johnston has, however, 

 on plates iv., and v., op. cit., figured fourteen species, and attached specific names 

 to six of them ; but, being undescribed, it is doubtful if any specific identifications 

 are possible, either from the impeifection of the portraitures or by the omission of 

 characters neces.sary to their correct classifactory position. Nevertheless the 

 resemblances which some of the Larapintine fos.sils bear to the less indistinct of 

 Mr. Johnston's figures lead to the belief that a considerable comnmnity in species 

 will eventually be found to obtain. 



The proofs are not conclusive, but there is presumptive evidence that the 

 Gordon River Group and the Larapintine Series are cotemporaneous and younger 

 than the Victorian Graptolite Slates. Mr. Johnston, op. cit., p. 51, inclines to 

 regard the Gordon River Group as the equivalents of the Caradoc Series of 

 England. So do I in respect of the Larapintine Ordovician, not so much because 

 of its cotemporanity with the Gordon River Group, but rather because of its 

 representative fauna. 



DESCRIPTION OF PLATES. 

 Plate I. 



Fig. \.—Orthoceras cheivingsi, n. sp. Natural size. 



,, 2. — Actinoceras iatei, Eth., f. «, natural section, showing septa and 

 siphuncle ; b, end view of septal surface. 



* Johnston, "Gcol. Tasmania," pp. 02-63. 



