from the Inferior Oolite of the Cotteswolds. 251 



when the mantle ceases altogether to secrete ornaments upon the 

 surface ; in the latter case a change occurs (sometimes sudden), 

 in which the carinae upon the area, and the tubercles upon the 

 sides of the shell simultaneously disappear, the last-formed por- 

 tion of the surface being altogether destitute of ornament. 



The mineral character of the beds in which Trigonia occurs is 

 very various; clays, argillaceous limestones, chalk, calcareous 

 oolitic freestone, and shelly oolitic drift equally contain it, but the 

 latter kind of deposit does not seem to have been favourable for 

 its development ; for although specimens are abundant the size is 

 dwarfed, and by far the larger number perished in the earlier 

 stages of their growth. 



In England the oldest stratum which contains Trigonia is the 

 Lias of Yorkshire, which produces the T. literata. Phillips re- 

 cords it in the lower, and Williamson in the upper Lias, but the 

 same formation has not furnished a single example of the genus 

 throughout the middle and west of England ; in Switzerland and 

 Germany the upper Lias has five species, none of which have 

 been identified in England. In the Cotteswold Hills, Trigonia is 

 first found in the beds of ferruginous oolite which immediately 

 overlie the sands at the base of the formation, and which abound 

 with Ammonites, Belemnites and Nautili, but the Trigonia are not 

 numerous, and are only of three species ; in the freestone beds 

 higher in the series, and which are so largely developed in Glou- 

 cestershire, some local deposits have many species of Trigonia, 

 but the genus does not acquire any particular prominence ; it is 

 only upon reaching the ragstones of the upper division of the 

 formation that we find Trigonia in abundance ; there it is asso- 

 ciated with a large assemblage of bivalve mollusks, and less 

 commonly with Echinodermata and Corals, but in either case the 

 impressions of Trigonia often constitute a large proportion of 

 the entire mass of the rock. 



The Inferior Oolite, in common with the middle and lower 

 Oolitic rocks generally, contains, according to the present ar- 

 rangement, two sections only of the genus Trigonia. 



In the following descriptions of species, the references to such 

 as have been before figured and described are given as concisely 

 as possible. 



COSTATJS. 



Trigonia costata, Lam. Var. 1. costata. Var. 2. mutticosta, 

 Var. 3. pulla. Var. 4. sculpta. 

 T. costatula, Lye. 

 T. exigua, Lye. 

 T. tenuicosta, Lye. 

 T. hnnispharica, Lye. 



