318 MISS MCDOXALD A^D DE. TRt'EMAX OX [vol. Ixxvii, 



three spirals, and those of the Inferior Oolite have four spirals ^ : 

 that is to say, in each case the ornamentation resembles that of the 

 particular group of Procerithium from which we suggest that the 

 pupoid form has been derived. 



(2) The ornamentation of these pupoid Procerithidae is frequently cata- 



genetic on the last whorl ; this was noticed in P. mimisynale. and is 

 also shown by many figured species called Exelissa by Hudleston 

 (9, pi. xi, figs. 5-9) and by Dr. Cossmann (3, pi. v). 



(3) These species are invariably small, a fact which further accords with 



their catagenetic natiu'e. 



(4) The pupoid form is due to a contraction of the whorl near the base, 



the last whorl having become cylindrical, with an entire and some- 

 what detached peristome. It is extremely likely that these features 

 are also catagenetic, the loss of the primitive canal being thus 

 easily explained; while Prof. Grabau has noticed that similar cha- 

 racters are to be found in catagenetic gastropods at various horizons," 

 and probably in phylogerontic Fusidfe, where the inner lip often 

 becomes detached from the columella and the whorl becomes some- 

 what cylindrical.^ 



On the other hand, it should be pointed out that there are 

 certain species of Froceriiliuim (and of Zygoplev.ra^ Avith cata- 

 genetic ornament which do not become pupoid, and there are also 

 pupoid species in other families in which the ornamentation shows 

 no trace of catagenesis ; these facts do not appear to us to be 

 seriously out of harmony with our suo-o-estion. for amonsr other 

 groups, such as the Ammonites, catagenesis rarely affects all the 

 characters at the same time. 



If our suggestion, that these species do not constitute a natural 

 group, should prove to be correct, the name Exelissa must not be 

 applied so widely as has hitherto been the case, and in the naming 

 of these forms two alternatives present themselyes : either new 

 generic names must be proposed for each catagenetic offshoot from 

 Froceritliiitm, or the whole of the species must be retained under 

 the name JProcerithium. This latter course has been adopted, since 

 our kno\vledge of these forms is not sufficientlj'' complete to allow 

 of detailed classitication, eyen if such detail is desirable at present. 



Proceeitiitum liassictm (Tate) nonlslooYe. (Text-fig. 6^, p.807.) 



CeritJiinm Jiassicum Tale (24, pi. ix, fig. 18). 



Cf. C. Uassicinn (Moore) (12, p. 20l'& pi. iv, figs. 16-17). 



Cf. C. ilminstevense (Moore) (l2, p. 200 & pi. iv, fig. 12). 



Dimensions of holotype (estimated). 



Length. Breadth. Length of spire. 



7'5 mm. 45 per cent. 66 per cent. 



Holotype : A specimen in the Tate Collection. Museum of Practical G-eology. 

 Jermyn Street, London, ISTo. 7968. 



This form apj^arently differs from Moore's specimen in being 



^ 9, pi. xi, figs. 5-7. 



^ A. W. Grabau, 'Principles of Stratigraphy' 1913, p. 973. 



'^ Id., 8, pp. 5, 96. 



Spiral 



Sufural 



angle. 



angle. 



30° 



108= 



