16 BRITISH FOSSIL TRIGONIiE. 



that the species occurs throughout the upper beds of the Inferior Oolite in the 

 Cotteswolds. 



Dimensions of the Hj/de specimen. Length ^\ inches; height 25- inches. 

 Dedicated respectfully to Sir W. V. Guise, Bart., F.L.S., F.G.S., of Elmore Court, now 

 and for many years President of the Cotteswold Naturalists' Field Club, a position 

 which he occupies with so much ability and benefit to its members. 



TiiiGONiA Walfgrdi, Li/cctt. Sp. uov. Trigon. Supplement, Plate II, fig. 8. 



Shell somewhat thick in the adult condition, ovately trigonal, moderately convex, 

 umbones prominent, recurved ; antero-mesial hinge-border comparatively short, obliquely 

 sloping ; siphonal border nearly as long as the hinge-border, its posteal extremity is some- 

 what produced and pointed ; the pallial border is much lengthened, and curved ellipti- 

 cally throughout its length without angularity. The surface of the area is somewhat 

 more raised than the other portion of the valve ; it is comparatively narrow ; its bounding 

 carinse are only slightly defined ; it has the usual median furrow distinct throughout its 

 length ; its surface is inconspicuously striated transversely, excepting near to the apical 

 extremity, where the bounding carinas are distinct. 



The surface has a few acute transverse costellae, which are equal in size to the first- 

 formed apical costa?, from which they are only separated by the minute carinal tubercles. 

 The escutcheon is rather wide and flattened, having rugose, oblique plications of growth. 

 The other and much the larger portion of the valve has the costse numerous (about 

 eighteen or nineteen), the few first-formed are narrow, closely arranged, and transverse or 

 transversely concentric, those which succeed change in figure and acquire tubercles at 

 their middle and posteal portions, so that at about the ninth costa the posteal portion of 

 the row has short tubercular varices, which commence an upward flexure to the marginal 

 carina ; with each succeeding costa these tuberculated flexed varices rapidly enlarge, but 

 with some irregularity, caused by the increasing prominence of the varices, so that the 

 ornamentation ^then acquires an aspect confusedly and prominently nodulous over the 

 middle and posteal portions of the valve ; the costae at the anteal portions continue 

 narroM^, rather acute, and attenuated even to their terminations at the pallial border, their 

 number exceeding by two the posteal large flexed varices. 



It will thus be seen that the costae undergo changes in their figure and ornamentation 

 throughout the entire growth of the shell, and that the general aspect of the surface would 

 scarcely be known or appreciated if illustrated by examples of less mature growth, or less 

 well preserved. Apparently the figured specimen is of adult growth, judging from the 

 plicated aspect of the siphonal border, and from the costae having almost ceased to form 



