298 MISS MCDOXALD AXD DR. TEUEMAX OX [vol. Ixxvii, 



many of the genera that had been proposed by earlier workers, 

 and has shown that many of the older names cannot be applied :<o 

 broadly as has been customar3\ 



Very complete evolutionary studies of smaller groups have been 

 published by Prof. A. W. G-rabau ^ ; while Miss Elvira Wood's 

 work on the Ceritliuim group ^ is of similar nature, and will be 

 referred to again later. 



In the present paper it is proposed to consider the evolution of 

 those slender Liassic gastropods which were formerly referred to 

 the followino; o^enera : 



Turritella, ■\vhicli included shells smooth, or with spiral ornament predo- 

 minant ; aperture holostome. 



Chemnitzia, with axial costas, spiral ornament feeble or absent ; aperture 

 holostome. 



Ceritliium, "with both spiral and axial ornament j^resent, these combining 

 in many cases to produce granules and tubercles ; aperture 

 generally with a canal. 



While these divisions may be recognized in a general Avay among 

 recent Gastropods, the separation of Liassic species into three such 

 groups is attended Avith considerable difficult}' ; most palaeontolo- 

 gists in the past have attempted to classify them in this wa}', and 

 the result in many cases has been confusing and unsatisfactory. 

 This is chiefly due to the fact that the separation has been based 

 largely on the form of the aperture,, and such distinctions are 

 difficult to apply, because — 



(1) Apertural margins are rarely shown in fossil specimens. 



(2) The Liassic species are so primitive that the characters in the aperture, 



which distinguish later forms, have not yet been evolved. 



The Liassic species which were referred to Chemnitzia and 

 Turritella are holostome, like the recent species of the corre- 

 sponding groups. The Liassic species referred to Cerithiiim, 

 however, are also generally holostome. Dr. Cossmann, among 

 others, has pointed out that, although many of the Liassic sj^ecies 

 of the Ceritliium group have no canal, some develop a feeble 

 sinuosity on the lip, which in later Liassic and Inferior Oolite 

 species has become a slight canal.-^ These species are sej)arated 

 with little difficulty from the other groups, when Avell-preserved 

 specimens with oral margins complete are available ; but it is clear 

 that the difference in aperture, which may be regarded as a differ- 

 ential character in recent forms, is not so apparent in Liassic 

 sjDecies. 



Some workers, realizino- the difficultv of classifvino- bv the form 

 of the apertm-e, have sought for other differential characters. 

 Among these workers we ma}^ mention Terquem, who, paying- 

 attention chiefly to the form of the columella, found that it is 



1 8. The numerals printed in thick type refer to the Bibliography, 

 § yill, p. 340. - 29. 



'^ See, for example, 2, vol. vii (1908) pp. 2-3. 



