86 BRITISH FOSSIL TRIGONIiE. 



T. gibhosa in the work of De Loriol and Pellat is exemplified only by the 

 variety a. 



The T. (jihhosu of Seebach (' Der Hannoversche Jura/ tab. 2, fig. a, G), from the 

 Pterocera Beds of Tongesberge (a lower zone of the Jura formation), is evidently a 

 distinct species characterised by a more trigonal form ; by the absence of the large 

 longitudinal sulcations and of the wide ante-carinal space, the costae are also different. 



Stratigraphical postion and Localities. In Britain T. gihbosa is the most well-known 

 Trigonia of the Portland Oolite, and is limited to that rock and the subjacent sands ; at 

 Chilmark and at Tisbury all its varieties are exemplified throughout beds, the entire 

 thickness of which is not less than sixty feet. Other reputed localities are the Isle of 

 Portland, Devizes, Brill, Hartwell, and Swindon, but the examination of a multitude of 

 examples of the GlabrcB from those places demonstrate that T. Dasmoniana is their 

 prevailing Trigonia. 



Chicksgrove Quarry, Tisbury, has disunited valves of T. gihbosa in great profusion, 

 sometimes covering large slabs of the bed called Troughstone, to the exclusion of all other 

 testacea. The Portland formation m Britain is well characterised by the presence of its 

 Trigonice, all of which appear to be special to it ; those of the oolites and sands are 

 T. Damoniana, T. Manseli, T. Micheloti, T. tenuitexta, T. miiricata, T. incurva, and T. 

 Carrei ; those of the lower or Kimmeridge Clay are T. VoUzii, T. Jaddiana, T. Pellati, 

 T. Woodwardi, also another lengthened form, somewhat doubtful, which I have provision- 

 ally placed as a variety of T. incurva (PI. IX, fig. 2). 



In the vicinity of Boulogne T. gihbosa is also an abundant species at numerous 

 localities, and in various beds of the upper portion of the Portland Formation. For 

 ample details the reader is referred to the important memoirs by M. Hebert and by 

 De Loriol and Pellat above cited. 



Trigonia Manseli, Lycett, sp. nov. PI. XIX, figs. 3, 4, 4 a, h. 



Shell subovate or ovately oblong, inflated mesially, compressed near to the pallial 

 border ; umbones antero-mesial, prominent, large, and obtuse, much incurved and nearly 

 erect ; anterior and lower borders curved elliptically ; hinge-border rather convex, curved 

 gently with the posteal extremity of the area and terminating in an extremity which is 

 somewhat produced and pointed. Escutcheon smooth and concave, but having its upper 

 border somewhat raised. Area narrow, convex, and raised, divided conspicuously by a 

 deep mesial furrow, which has bordering upon it, immediately upon either side, a slightly 

 defined row of small tubercles, or in other specimens they are evanescent ; there is also a 

 well-defined line of small tubercles or varices which forms an inner carina ; these varices 

 are extended somewhat upon the escutcheon. The position of the marginal carina is 



