﻿Vol. 66.^\ SPECLES OP AMMONITES AND BRACKIOPODA. 91 



hundred hemerae. Therefore, according to this reckoning, the few 

 feet of Inferior Oolite represent from about a quarter to a fifth of the 

 total time occupied in the deposition of the entire Jurassic System. 



One can hardly view the few feet of Inferior Oolite limestone at 

 Burton Bradstock, about 15 to 20 feet say, and imagine that it 

 represents an interval of time equal to a quarter or a fifth of 

 the whole Jurassic Period — a time during which thousands of feet 

 of strata were laid down. But this is because we do not allow 

 sufficiently for the gaps. 



If anything like this supposition be correct, then the Inferior 

 Oolite prolificness is understandable : it should represent in species 

 as many as would form from a quarter to a fifth of the total for 

 the Jurassic System. At that rate, its tale is nothing like complete 

 yet ; which all workers in its rocks know to be the case. 



For most of the photographs of Ammonites which illustrate this 

 paper, and for all the photographs of Brachiopods, I desire to 

 express my best thanks to my enthusiastic friend, Mr. J. W. Tutcher, 

 whose excellent work I have acknowledged on other occasions ; 

 but all such acknowledgments and thanks fall very far short of the 

 measure of my indebtedness to him. 



In the descriptions of some of the following species of Brachio- 

 poda, the consequences of a former lack of zonal divisions of strata 

 will be noticed. Because the top beds of the Inferior Oolite — from 

 Garanticma to fusca in some cases, from truellii to zigzag in others 

 — were called simply parhlnsoni zone, the dates of some species 

 are uncertain within these limits : that is to say, which species are 

 truly contemporary and which are successive, was not properly 

 noted at the time of collecting, because the strata were grouped too 

 comprehensively. 



II. Descriptions of Ammonites. 



Family Amaltheidse (subfamily Sonnininae). 



Genus Shirbuirnia, 1 nov. 



Definition. — "Whorls stout, trigonal, with considerable in- 

 clusion. Ornament coarse, but showing rapid decline to levigate 

 stage. Carina distinct, not large, hollow. Suture-line not com- 

 plex ; superior lateral lobe broad-stemmed, short. Lobules short, 

 inequipoised ; terminal lobule endobraehysceles ; inner lobes, not 

 dependent. 



Distinction. — From Sonninia (S. propinquans series), the stout 

 whorls, the rather small carina, the not complex suture-line, the 

 broad-stemmed superior lateral lobe. From Dorsetensia, of which 



1 Sherborne (Dorset), where the species of the genus are found ; but I would 

 also like to associate with the name my kind friend, Mr. C. Davies Slierborn, 

 as a small expression of thanks for his invaluable bibliographic assistance on 

 many occasions. 



