﻿COSTAT/E. 



177 



feet in thickness have produced a greater number of specimens of T. hemisphcerica than 

 >the whole of the Cotteswolds. As the geology of the vicinity of Appleby was but little 

 known previously to the memoir by the Rev. J. E. Cross (' Quart. Jour. Geol. Soc.,' May, 

 1875), my thanks are especially due to that gentleman under whose guidance I visited 

 the quarries of that vicinity in the year 1870; the specimens figured in this monograph 

 were the gift of Mr. Cross. At Cloughton upon the coast to the northward of Scar- 

 borough, the Inferior Oolite Millepore bed contains some dwarfed examples of the variety 

 gregaria associated with T. recticosta ; owing to the hard matrix and delicate condition 

 of the testacea ; specimens are rare and usually ill-preserved, they measure from six to 

 ten lines across the valves. 



Affinities. — The distinctive features of T. hemispharica are so conspicuous, that it 

 will not readily be mistaken for any other species. The only form in the lower oolites 

 which appears to approach to it is T. tenuicosta ; when comparing specimens of equal 

 dimensions it will be seen that T. tenuicosta has greater convexity, that it is shorter and 

 truncated anteally ; its umbones are more elevated, narrow and pointed, its area is much 

 more excavated ; its carinas and the surface of the area generally has a much smaller and 

 more delicate ornamentation ; the costae also have a considerable anteal undulation 

 different from the simple anteal curvature of T. hemisphcerica. 



In the Upper Jurassic rocks of Erance one of the Costata resembles T. hemisphcerica 

 in having very numerous small costae; T.Etalloni, DeLor. (' Paleont de la Haute Marne,' 

 P. de Loriol, et E. Royer et H. Tombeck ( f Mem. Soc. Lin. de Normandie,' torn. 13, 

 PI. XVII, figs. 13, 14, 15), has the numerous costae curved concentrically throughout 

 their course ; the marginal carina is minute, and therefore wholly unlike the large and 

 deeply sculptured carina of the British species. 



Our species is not limited to a single stratigraphical position of the inferior oolite. 

 It occurs in the middle portion of the range of the Cotteswold Hills, between Chel- 

 tenham and Stroud ; its bed (Lower Trigonia Grit) overlies nearly two hundred feet of 

 the Inferior Oolite. In North West Lincolnshire, at the Santon cutting, only four or 

 five feet separate the shelly bed with T. hemispharica from the dark clays of the Upper 

 Lias ; both this and the position of the Lincolnshire Limestone are widely separated 

 from the Trigonia bearing Ragstones of the Cotteswolds ; the latter occupy a much 

 higher position, and have their ornaments more prominently sculptured than the 

 Lincolnshire specimens, in every stage of their growth. 



The steep escarpments upon the eastern side of the Naiisworth Valley, with its 

 numerous quarries and other smaller rock exposures, has in its beds of hard white 

 limestones and their enclosed Conchifera exact counterparts of the fauna and deposits of 

 North Lincolnshire, reproducing upon a smaller scale the limestones which, in their 

 more northern prolongation, acquire so much greater thickness and importance. The 

 bed of oolite marl has disappeared in the Naiisworth valley, merged in the more indu- 

 rated limestones which are altogether without T. hemisphcerica. 



