spiral 



Suiural 



angle. 



angle. 



20^ 



115° 



32-dt MISS MCDONALD AIS'D DR. TEr£MA^^^ ON [vol. Ixxvii, 



(C) Paraceeithium Cossmann. 



Shell fairly stout, aperture oval, terminated by a short projection ; 

 spire very heavily costulate, with spines near the posterior margin. 



Paraceeithium sp. (PI. XXII, fig. 9.) 



Dimensions of our specimens. 



Lengtli. JBreadth. Lengtli of spire. 



17 mm. 40 per cent. 64 per cent. 



Short spinous forms not sufficiently well preserved to show 

 development, though it appears that the ornamentation on the early 

 whorls consists of simple axials. The axial ribbing of later whorls 

 is very pronounced, not more than ten or twelve to a whorl, with 

 wide interspaces and projecting tubercles near the posterior suture, 

 giving to the shell a stepi^ed appearance. 



As Cossmann's classification suggests, I^ a race n't Jiinm belongs to 

 a different group of the Procerithidae from Procerifhium ; probably 

 it is more ancient (3, pp. 1-8). We have been unable to study any 

 examples of Paraceeithium earlier than this Toarcian form, and 

 therefore we are unable to suggest with any certainty what may 

 have been the line of evolution of the Paraceritliium group. The 

 species of ParaceritJiiiim figured from earlier zones in the Lias 

 by Dr. Cossmann, however, indicate that the genus was probably 

 evolving slowlj^ : ioY msi2iiiQe, P. ferrenchim Cossmann, from the 

 Charmouthian, is not verj^ different from the species that we have 

 been describing. 



It is not unlikely, however, that some of the spinous species 

 referred to Paraceritliium are merely sharply tuberculate species 

 of ProceritJiium. 



Locality and horizon. — Our specimens were found in the 

 Oolite ^Q(\., faJ cifer zone, of the LTpper Lias, Grantham. ^ 



Y. Family of the LoxoNEMATiDiti Koken. 



This family includes gastropods from Palaeozoic as well as 

 Mesozoic rocks. The Liassic species are generally simple in form 

 and ornamented by axials, which may or may not be crossed by 

 spirals ; whenever spirals are present, the}^ are almost always finer 

 than the axials, wherefore ' tuberculation ' is exceptional in this 

 family. These forms in the past were generally referred to Oliem- 

 nitzia ^nd Melania ; and, so far as we have been able to determine, 

 all are alike in having axial ornament first in development, and 

 a more or less oblique columella. In most forms the oral margin 

 is comparatively simple, the aperture holostome, and the lip more 

 or less sinuous. Dr. Cossmann, to some extent following Koken 

 and other workers, has subdivided this family into some fifteen 

 sections, not all of which, however, are found in Jurassic rocks. 



1 A. E. Trueman, ' The Lias of South Lincolnshire ' GeoL Mag. 1918, p. 107. 



