810 MISS MCDOISMLD AXD DE. TEt'EMAX OX [vol. IxXVU, 



So far as we have been able to determine, the columella m all 

 these forms is straight. 



In development, the various species are all alike in having smooth 

 embr^'onic whorls, the first ornament to appear being a single 

 median spiral, to which are subsequently added other primary and 

 secondary spirals. Axial ornament never develops until after the 

 appearance of one or more spirals. 



Most of the species that we have examined are here referred to 

 the genus ProceritJiiiim; Dr. Cossmann subdivides this genus 

 into numerous sections, of which Cosmoceritliium, Bhahdocolplius, 

 and XystreUa may be mentioned. 



Of the other genera named by Cossmann under this family we 

 have examples (A ^araceritliium, which have been studied; this 

 genus comprises subspinous forms in which tlie spirals are often 

 faint, and the axials produced to a low tubercle on the posterior 

 maro;in of the whorl. Similarlv CerithineUa shows a charac- 

 teristic type of ornamentation, with rows of tubercles near the 

 suture. Probably these genera are correctly defined, because tbe 

 style of ornamentation is sufficiently marked to serve as a real 

 indication of relationship ; the genus Crijptaulax may, perhaps, be 

 admitted for the same reason. 



On the other hand, the genus ExeJissa, proposed by E. Piette 

 to include forms in which, owing to the contraction of the 

 later whorls, the shell becomes pupoid, probably is not a natural 

 group ; this question is fully considered later (p. 317). 



The genera of the Liassic Procerithidse dealt with in this paper 

 are, therefore, as follows : — - 



Proceritldvm. Spirals and axials forming' a network (often with tubercles). 



The spirals are, for the g'reater part, equally spaced. 



The oral margin shows a simple sinuosity. 

 CerithineUa. Whorls generally flat, ornamented by tubercles arranged 



in definite bands, often separated by a smooth space. 

 Pavaceritliiiim. Spire strongly ornamented with axials and sharp tubercles ; 



the oral margin shows a stronger projection than Pw- 



ceritMtim. 



(A) Proceeithiijm Cossmann. 



This includes many Liassic Cerithium-like gastropods. Several 

 groups may be recognized among the English forms that we have 

 been able to study ; but, beyond suggesting these, we do not 

 propose to introduce any rigid scheme of generic classification 

 based on present knowledge. It is certain that, despite the great 

 number of species already described, there are still many more as 

 yet unfigured ; and, until more of these are known, it would merely 

 cause confusion in the future to indicate new genera. 



Tlie chief group in the lower part of the Lower Lias consists 

 of small forms, such as JProceritliium ogerieni, P. ahcist()?i, etc., 

 which have rather flat whorls, separated by deep wide sutures, and 

 ornamented by three spirals crossed by axials, accompanied b}^ the 

 formation of a network Avith low tubercles at the intersection. 



