340 MISS MCDOXALL) A>D UE. TEUE]\rA>' O'N [vol. Ixxvii, 



VII. GeOLOUICAL DlSTKIBTJTIOy. 



The present investigation was undertaken, chietiy in order to 

 determine the value of Gasti-opods in the correlation of Mesozoic 

 rocks. In the zoning of Jurassic rocks the Ammonites have 

 hitherto proved most useful ; but it is well knoAvn that in some 

 divisions of the Jurassic rocks ammonites are rare, and in such 

 cases greater attention must necessarily be paid to other fossils. 

 Xaturally, those organisms are most useful in correlation that 

 have evolved rapidlv in some definite direction, and it will there- 

 fore be convenient to consider briefly the evolution of the two 

 most important families with which we have been concerned : 

 namely, the Procerithidse and the Loxonematidai. 



(1) Procerithidse. 



The Procerithidse are very common in the Liassic rocks of this 

 coimtry, the genus Froceritliium being particularh' abundant. 

 In England other genera such as Crypiaulax, Ceritliinella, and 

 Paraceritliium do not appear to be by any means common. , 

 Procerithium is found somewhat sparingly in Hettangian rocks, 

 m.ore commonl}^ in the Sinemurian, and apparently becomes most 

 abundant in the Charmouthian. At some horizons in the Middle 

 and Upper Lias, notably in the acuiiim zone of Leicester, Pro- 

 ceritliium asrain becomes common. In the lowest zones of the 

 Lower Lias the predominant species of Procerifliium have some- 

 what flat whorls, ornamented b}" a network based on three spirals, 

 and these species may be considered as constituting the central 

 stock of Proceritliium in the Lias ; one of the most typical species 

 is -P. ogerieni. From them evolved numerous offshoots, some by 

 the addition of spirals, others by the accentuation of a spiral 

 forming a carina, others by differentiation of the ornamentation. 

 This central stock presumal)ly existed with little modification 

 throughout the Liassic Period, and gave rise to species of the 

 Procerithium-miiricatum type, which are the dominant forms in 

 the Inferior Oolite. ^ This type resembles very closely the P. oge- 

 rieni-ij'pe of, the Lias, of which it is probably a descendant ; but 

 it has progressed in size, in the development of a canal, and in the 

 addition of a fourth spiral. 



We may, therefore, note that, while the main stock of Pro- 

 ceritlihim apparently progressed slowly throughout Jurassic times, 

 yet there were numerous species of Proceritliium that maj have 

 been offshoots from the central stock, in which evolution w^as much 

 more rapid. 



(2) L(jxonemati(l3e. 



The Loxonematidse are chiefl}^ represented in the British Lias 

 by two main divisions, Zygopleura and Katosira ; they are ex- 

 tremely common in the Lower Lias, but are rare in the Middle 



1 9, p. 145. 





