THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. II. 



No. X.— OCTOBER, 1885. 



oie.iC3-iisr-A.Xj .A.x^TIGXJ:ES. 



L — On Eeoent and Fossil Pleubotomarim. 



By Henry Wdodward, LL.D., F.R.S., F.G.S. 



(PLATE XI.) 



THE progress of modern discovery, both in Paleeontology and 

 Zoology, has been steadily tending to bridge over the gap 

 v^hich seemed at one time to separate the past from the present 

 life-history of our globe, and has brought each more closely into 

 relationship with the other than seemed possible fifty years ago. 



Palaeontologists are constantly discovering fossil forms to connect 

 the past with the present, and zoologists, with equal zeal, are seek- 

 ing new living ones to connect the present with the past. 



The discovery of Trigonice in the seas, and the existence of Ceratodus 

 in the rivers of Australia, offer us living analogues for a long sei'ies 

 of ancestral forms, carrying us back through all the Secondary strata 

 to the Trias, and in the case of Ceratodus, to the Coal-period ; whilst 

 the Lingnla and the Pearly Nautilus, living to-day, have found 

 remote ancestors in Silurian and Cambrian times. 



The almost simultaneous discovery of remains of Scorpions in 

 the Silurian rocks of America, Scotland, and Sweden, closely re- 

 sembling those now living, attests the presence of dry-land and 

 Insect-life in pre-Carboniferous times, and furnishes another link in 

 the " Enchainements du Monde Animal," by which the long-buried 

 past lives again by the light of the life of the present day. 



A similar interest, zoologically and palseontologically, attaches to 

 the ancient genus Pleurotomaria, a Gasteropod which, previous to 

 1855, was only known in a fossil state. 



A large proportion of the older trochiform fossil shells have their 

 whorls, whether round or angular, marked by a peculiar band, 

 usually terminating in a deep slit at the aperture. Most of these 

 were solid nacreous shells, and have been referred to the genus 

 Pleurotomaria. Others are slender and tapering, and resemble a 

 Ceritliium with a notched aperture : they are named Murchisonia, 

 and are probably related to Pleurotomaria proper. 



In Woodward's "Manual of the Mollusca " the genus Pleurotomaria 

 is said to range from the Silurian to the Chalk formation, and the 

 number of species then recorded (in 1854) was 400. Since that 

 date the number of fossil forms described has been greatly increased. 

 The palgeontological gap which seemed to separate the fossil Pleuro- 



DECADE III. — TOL. II. — NO. X. 28 



