B. Tate — Census of the Lias Invertehrata. 5 



necessarjy chiefly because, from the great number of separate palaeon- 

 tological memoirs extant, estimated at upwards of 130, authors are 

 liable, from inadvertency, to describe as new what may have already 

 been figured. The variability of certain species is also a fruitful 

 source of multiplication of specific names ; . and the synonj'^ms of 

 certain species, whose limits are ill-defined, are most voluminous; 

 such are Cardinia ovalis and others, Pecten dextilis, Rhynclionella 

 variabilis, Spiriferina rostrata, etc., etc. 



My own labours have been more especially directed to the 

 MoUusca, but much remains to be done before an approximate 

 estimate of the number of species belonging to this sub-kingdom 

 can be given. Several species not included in my census have been 

 described, the claims of which to a place in a catalogue can onty be 

 determined by an examination of the type specimens. The deter- 

 mination of such dubious species will necessitate a visit to several 

 public museums. Thus, the Whitby Museum contains the types of 

 130 Cephalopoda, and 122 species of other classes of Mollusca, the 

 names of which have not been generally accepted. Indeed, with 

 the exception of the Cephalopoda, very little is known respecting 

 the species of the Yorkshire Lias, and till we know more of them 

 and of their distribution, the sub-divisions of the Liassic rocks of 

 Yorkshire cannot be brought, with any degree of confidence, into 

 juxtaposition with those of the more typical district of the south- 

 west of England, 



Bistrihution of the Genera. — In the accompanying table are given 

 the names of all the genera known in the Lias, and the numbers in 

 the first column refer to the number of species of the genus in the 

 British Lias, whilst those in the second column indicate the total 

 number of species, British and Continental. 



The Ehsetic species are excluded from these enumerations, as we 

 are in possession of full information respecting them ^ ; but the 

 species from the Supra-Liassic Sands are included. 



The total number of marine invertebrate species in the European 

 Lias is 2,126 ; of these 997, or nearly one-half, are found in Great 

 Britain. Singularly enough, this proportion holds good for the 

 majority of the classes (see Summary, Table II.). 



If to the number of marine invertebrate fossils we add those of 

 the Foraminifera, insects, plants, and the vertebrata, we have a total 

 of 1,228 species in the British Liassic deposits. 



Class Cephalopoda. — All the genera are, at least, Mesozoic ; one 

 only, Xipthoteuthis, is peculiar. 



Class Gasteropoda. — A large proportion of the genera date from 

 the Trias, such are Acteonina, Chemnitzia, Cerithium, Fusus, Neritopm, 

 etc. ; a few range upwards from the older rocks, as Chiton, Dentalium, 

 Litorina {?), Patella, Natica, Pleurotomaria, Turho, and Trochus {?) ; 

 Cryptcenia became extinct in the Middle Lias; Alaria and Trochotoma 

 commence in the Kh^tic ; whilst Bulla, Cryptolax, Exelissa, Onustus, 



1 M. Jules Martin, E'tage Ehfotien. 



