Prof. F. M'Coy on a Tertiary Pleurotomaria. 101 



XT. — On a Tertiary Pleurotomaria. By Frederick 

 M'Coy, Professor of Natural Science in the University 

 of Melbourne. 



To the Editors of the Annals and Magazine of Natural History. 



Gentlemen, 



In former numbers of your Journal I have made known the 

 existence in the Tertiary rocks of Victoria of three species of 

 Trigonia 1 previously only known as an abundant Mesozoic 

 genus, and represented by a few living species in Australasian 

 seas, but, by its complete absence in the intervening Tertiary 

 periods, forming a remarkable exception to the ordinary rule 

 of the duration of a genus in time being always continuous. 

 I have now the pleasure to announce a parallel discovery, 

 which will, I have no doubt, be of interest to geologists and 

 also to zoologists occupied with the general question of the 

 duration of genera, as removing another conspicuous supposed 

 exception to the general law. 



The genus Pleurotomaria, like Trigonia, is a most abun- 

 dant one in all the Mesozoic marine formations, but, like 

 Trigonia also, has hitherto been remarkable for its sudden 

 disappearance at the close of the Cretaceous period and 

 being entirely absent in the well-searched Tertiary forma- 

 tions of Europe, Asia, and America, but reappearing in our 

 recent seas, where it is represented by two excessively rare 

 species. 



I have recently found in a hard brownish or yellow lime- 

 stone, which, from its other fossils, is undoubtedly of the 

 Upper Miocene Tertiary age, a fine large species of Pleuroto- 

 maria almost intermediate in character between the two living 

 ones, having the large size, more elevated spire, and more 

 numerous and flatter whorls of the living Pleurotomaria Adan- 

 soniana, but the more nearly central band of the P. Quoyana, 

 and having the close spiral thread, crossed by nearly equally 

 prominent equidistant lines of growth. It differs from both 

 in its more elevated acute spire, or much smaller apical 

 angle, and more equal stria). It may be characterized as 

 follows : — 



Pleurotomaria tertiaria (M'Coy). 



Spec. char. Shell large, trochiforai, apical angle 67° : whorls 

 flat or very slightly convex ; base moderately convex, with (?) 

 a small umbilicus ; band of moderate width, in the middle of 

 each whorl, slightly depressed : surface with subequal promi- 



